Hey BD,
A couple things to consider:
First, are you using wire or braid? Wire counts a bit more accurately than braid.
Secondly, are you sliding the counter out to "zero" it occasionally?
How much wire is on the spool? Low spool readings are notoriously inaccurate!
Finally, make sure the wire is not stacking up on the spool, causing a bad reading. Use the ball hook for a fair lead and keep the wire centered in the spool and you'll have more accurate readings.
Good luck and I hope this helps!