Troubles dialing in A axis

The prototype I am building utilizes Mach3, Gecko 320X drivers and very small servo motors. The A axis is controlling a knife. I am using tangential control to position the A axis. The gear train utilizes a 20:1 reduction worm/worm gear.

When I cut a series of 1" diameter circles the DRO never returns to the starting point, which in this case is 270.0. The DRO increments in steps of .0010" after each circle is cut (270.0000, 270.0010, 270.0020...). At the end of the series cut, when the tool is homed, the blade is always misaligned from zero.

I've tried to adjust the "Steps per" setting under Motor Tuning. Also, I've determined and set the appropriate backlash value for that axes gear train (though backlash shouldn't come into play because the axis never changes direction with a circle).

I don't understand why the DRO increments in .0010".

Suggestions?