Step count / inch problem

I have a question about why the math did not work out for my step
counts per inch. On my servo system I just finished and using the
calculations I got from help here, see below

servo with 250 cpr, .2 pitch ball screw, 2:1 pulley system.

250 x 4 x 5 x 2 = pulses per inch or 10,000

When I place this figure into my controller software and test it with
a dial indicator. (backlash taken into account) I come up a few thou
short. and a little more when I move 2". I had to set the counts in
the controller software to 10,010 to make it come out dead nuts on at
1" and very close at 2" (This was the X axis on the mill)

When I checked the Y axis I only needed to alter the value to
10,005. The Z was fine at 10,000. What would cause this? Could it
be that I did not purchase expensive ground ball screws?

Jeff Davis
http://www.homecnc.info