Rectangular slots undersized

I understand why interior circles come out undersized, but is there a reason for interior rectangles to be undersized? The slots in this part are designed to be 5mm, but print at +- .1mm of 4.5. The X and Y are clalibrated to be within abut .05mm of each other, and the extruder is calibrated as well, so that when I print a single perimeter wall, the thickness of the wall matches the thread width that has been set (1.4 W/T or .42). Both slic3r and skeinforge have this issue though slic3r seems to be better.



Does the slicer need to recognize that they are interior slots to place the correct side of the thread on the face? It seems fairly cut and dried with this model.

Have you verified that the layer width you get is what you expect with a single walled test print?

Yes, the single wall is within .02mm or so of the thread width I entered into Slic3r, I had to set the extrusion multiplier to .92 to get it there. I'm also being careful not to include the first layer when measuring since it is a little more squished.

Which version of Slic3r?
There were bugs in the 0.8x releases that could cause this.

Slic3r 0.9.1. I can try Skeinforge, but I don't have it dialed in as well as Slic3r. Maybe the bug is still there in the new version.