Auto bed leveling working as it should?

I have an apparently working auto bed leveling setup on my old i2. I'm however wondering if it works at all. I've only printed very small objects, and now than I'm printing slightly larger ones, I'm finding that the Z offset is spot on in the area I usually print, but lower down, it is too high. This results in warping. Should not this exact problem be something my auto bed leveling can compensate for?

This is the result of my G29 command:

SENDING:G29
Bed x: -20.00 y: -10.00 z: 22.25
Bed x: 120.00 y: -10.00 z: 22.32
Bed x: 120.00 y: 125.00 z: 22.62
Bed x: -20.00 y: 125.00 z: 22.51
Eqn coefficients: a: 0.00 b: 0.00 d: 22.27
planeNormal x: -0.00 y: -0.00 z: 1.00
echo:endstops hit: Y:-0.06 Z:22.51

Is my problem perhaps, simply that, since my Z offset is spot on at one area, but too high in others, that the Z offset will never adjust into a negative offset, and would only adjust for positive tilting -- i.e. if the offending area was _higher_ than the rest?

Or is my problem that I'm not using auto bed leveling in the correct manner? I simply do this as my custom start G-gode:

M80 ; PSU on
G28 ; home all axes
G29 ; auto bed leveling

Do I need more than this to activate the real time Z offset adjustment while printing? I don't believe I have any firmware or hardware issues, my firmware is as follows:

//============================= Bed Auto Leveling ===========================

#define ENABLE_AUTO_BED_LEVELING // Delete the comment to enable (remove // at the start of the line)
#define Z_PROBE_REPEATABILITY_TEST  // If not commented out, Z-Probe Repeatability test will be included if Auto Bed Leveling is Enabled.

#ifdef ENABLE_AUTO_BED_LEVELING

// There are 2 different ways to pick the X and Y locations to probe:

//  - "grid" mode
//    Probe every point in a rectangular grid
//    You must specify the rectangle, and the density of sample points
//    This mode is preferred because there are more measurements.
//    It used to be called ACCURATE_BED_LEVELING but "grid" is more descriptive

//  - "3-point" mode
//    Probe 3 arbitrary points on the bed (that aren't colinear)
//    You must specify the X & Y coordinates of all 3 points

  #define AUTO_BED_LEVELING_GRID
  // with AUTO_BED_LEVELING_GRID, the bed is sampled in a
  // AUTO_BED_LEVELING_GRID_POINTSxAUTO_BED_LEVELING_GRID_POINTS grid
  // and least squares solution is calculated
  // Note: this feature occupies 10'206 byte
  #ifdef AUTO_BED_LEVELING_GRID

    // set the rectangle in which to probe
    #define LEFT_PROBE_BED_POSITION -20
    #define RIGHT_PROBE_BED_POSITION 120
    #define BACK_PROBE_BED_POSITION 125
    #define FRONT_PROBE_BED_POSITION -10

     // set the number of grid points per dimension
     // I wouldn't see a reason to go above 3 (=9 probing points on the bed)
    #define AUTO_BED_LEVELING_GRID_POINTS 2


  #else  // not AUTO_BED_LEVELING_GRID
    // with no grid, just probe 3 arbitrary points.  A simple cross-product
    // is used to esimate the plane of the print bed

      #define ABL_PROBE_PT_1_X 15
      #define ABL_PROBE_PT_1_Y 120
      #define ABL_PROBE_PT_2_X 15
      #define ABL_PROBE_PT_2_Y 15
      #define ABL_PROBE_PT_3_X 120
      #define ABL_PROBE_PT_3_Y 120

  #endif // AUTO_BED_LEVELING_GRID


  // these are the offsets to the probe relative to the extruder tip (Hotend - Probe)
  // X and Y offsets must be integers
  #define X_PROBE_OFFSET_FROM_EXTRUDER -23
  #define Y_PROBE_OFFSET_FROM_EXTRUDER -3
  #define Z_PROBE_OFFSET_FROM_EXTRUDER -22.5

  #define Z_RAISE_BEFORE_HOMING 5       // (in mm) Raise Z before homing (G28) for Probe Clearance.
                                        // Be sure you have this distance over your Z_MAX_POS in case

  #define XY_TRAVEL_SPEED 4000         // X and Y axis travel speed between probes, in mm/min

  #define Z_RAISE_BEFORE_PROBING 25    //How much the extruder will be raised before traveling to the first probing point.
  #define Z_RAISE_BETWEEN_PROBINGS 10  //How much the extruder will be raised when traveling from between next probing points

  //#define Z_PROBE_SLED // turn on if you have a z-probe mounted on a sled like those designed by Charles Bell
  //#define SLED_DOCKING_OFFSET 5 // the extra distance the X axis must travel to pickup the sled. 0 should be fine but you can push it further if you'd like.

  //If defined, the Probe servo will be turned on only during movement and then turned off to avoid jerk
  //The value is the delay to turn the servo off after powered on - depends on the servo speed; 300ms is good value, but you can try lower it.
  // You MUST HAVE the SERVO_ENDSTOPS defined to use here a value higher than zero otherwise your code will not compile.

  #define PROBE_SERVO_DEACTIVATION_DELAY 300


//If you have enabled the Bed Auto Leveling and are using the same Z Probe for Z Homing,
//it is highly recommended you let this Z_SAFE_HOMING enabled!!!

  #define Z_SAFE_HOMING   // This feature is meant to avoid Z homing with probe outside the bed area.
                          // When defined, it will:
                          // - Allow Z homing only after X and Y homing AND stepper drivers still enabled
                          // - If stepper drivers timeout, it will need X and Y homing again before Z homing
                          // - Position the probe in a defined XY point before Z Homing when homing all axis (G28)
                          // - Block Z homing only when the probe is outside bed area.

  #ifdef Z_SAFE_HOMING

    #define Z_SAFE_HOMING_X_POINT (X_MAX_LENGTH/2)    // X point for Z homing when homing all axis (G28)
    #define Z_SAFE_HOMING_Y_POINT (Y_MAX_LENGTH/2)    // Y point for Z homing when homing all axis (G28)

  #endif

  #ifdef AUTO_BED_LEVELING_GRID	// Check if Probe_Offset * Grid Points is greater than Probing Range
    #if X_PROBE_OFFSET_FROM_EXTRUDER < 0
      #if (-(X_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
	     #error "The X axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
	  #endif
	#else
      #if ((X_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
	     #error "The X axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
	  #endif
	#endif
    #if Y_PROBE_OFFSET_FROM_EXTRUDER < 0
      #if (-(Y_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
	     #error "The Y axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
	  #endif
	#else
      #if ((Y_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
	     #error "The Y axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
	  #endif
	#endif

	
  #endif
  
#endif // ENABLE_AUTO_BED_LEVELING