backlash with usb motion card

hi experts,

i get a usb motion card via ebay. all works well. in/outputs and motion are ok. but when i measure my axis, i 'found' a backlash.
for example:

Y-axis is 0.
move Y to 0.55 an measuring show 0.47
move Y back to 0.00 measuring show 0.00
move Y to 0.55 an measuring show 0.47
move Y back to 0.00 measuring show 0.00
and so on

after this, i changed back to parallel port and all is ok - no backlash on axis
now i changed to usb motion card again and use a different PC - the same issue.
any hints...?