Try to calculate 750-693. You may see it returns incorrect value. I got 56. There are some more incorrect results.
Yesterday, VMichl notify me that this issue has been fixed in AutoCAD 2010 update 1.
Snipped from readme:
The following defects have been fixed:
(One of them is:)
Calculations performed by the QUICKCALC command may be incorrect.
If you have the same problem, download the update 1 here. Install it, and see if it’s fixed.