Tabbed box sizes not accurate

I am using Kiri:Moto with Onshape and a cheap 3018 desktop router. I am cutting light plywood using a 1/16" bit. For 99% of what I am doing this combination is very accurate. I have noticed an issue when creating “tabbed” joints (like from one of the online tabbed box generators, but same issue when I draw the joints myself). The issue is that the size of the tabs and slots isn’t accurate. For example, if I want a 20mm wide tab, it may be 20.3mm and the slot it is supposed to fit into is only 19.6mm. I always have to touch up the tabs and slots with a Dremel to get things to fit.

Is there a setting somewhere that I can compensate for this tendency? Or do I need to calibrate the machine and modify its settings? A recent part that was supposed to be 100mm x 40mm ended up 99.9mm x 40.3, so the calibration is off a little, but not by the amount of error I am seeing in the tabs and slots.

I believe the gcode output is accurate and it’s the machine that needs calibration. it’s seems entirely possible that the “firmware” steps per mm for one or both of your X,Y axes is inaccurate. Or your bit size is not properly configured in KM which would cause faces to be offset.