Hi maya647722 -- yes the tests can be so frustrating! The GMAT scores are actually not based on a pure right/wrong calculation, it's a complicated algorithm where the computer assigns problems of varying levels of difficulty. How you do on more difficult versus easier problems is how it calculates your final score. Also, the practice tests developed by the various companies are calibrated differently - none of them are an exact match to either the 'real' GMAT nor to each other, and so the same person often will have a very different score when completing the different companies' tests.

Hopefully this helps a little! It's unlikely that there's a bug on the Vista version that you're using but it's possible. It may be more due to the mystifying rules that the GMAT uses 'under the hood' for scoring.

EssaySnark