Homing the z-axis

Hi
I'm trying to figure out how z-homing works. For X and Y axis my printer check for endstop to switch, than came back and check it again. I can request to home these axis multiple time, but the printer act as described every time. For Z axis this is not true:
- With my extruder away from print plate (for security reason, the endstop is near half of Z rods) I set the actual point as Z-home.
- I move extruder 10mm up, then I ask Z to home.
- The extruder go down, hit the endstop and then lift again 1mm.
- If I ask Z to home again, extruder list a little, then go down until the endstop switch and here it remain.
- Every time I ask a z-home, the extruder switch from these two position.

I noticed this while bed leveling and I'm not sure what is my real z zero point.
Any suggestion?

I fount these line in configuration.h:

#define ENDSTOP_X_BACK_MOVE 5
#define ENDSTOP_Y_BACK_MOVE 5
#define ENDSTOP_Z_BACK_MOVE 1

I noticed that 1mm is not good to switch my endstop off again. Increasing it solve the problem. This option is visible only in expert mode, but I think the default value should be changed to 2mm at least.

I have the same behavior - inconsistent zero on homing. I can never set my Z zero correctly by adjusting the microswitch alone. I have to get it close then make adjustments in the Slic3r Printer Setting Z offset. I do several prints ( just running long enough to assure the layer sticks but not drags until I am happy . I broke a switch already because it drives down too far. I will check your suggestion and let you know.