I just wonder if you may have a metric lead screw in the "Y" axis and an inch scale on the hand wheel. This could explain the large difference.
I checked the threads per inch on the X axis and there are ten, as it should be. I then checked the threads on the Y axis, and there 13 threads per inch. So I set the handwheel at zero and switched the DRO to read millimeters.
1 rotation of the handwheel, table moves 2mm.
5 rotations and the table moves 10mm.
10 rotations and I get 20mm.
So, yes indeed, they put a metric screw on the Y axis.
I'm guessing that to correct this I will have to remove the table and the crosslide. Will I be able to access the "nut" by doing this, or is it accessed from underneath.