800 score test vs Manhattan free CAT test

Your confusion is coming from how the GMAT algorithm works. Your experience is complicated by having taken two tests from two different sources.

You could have experienced similar results even if you had taken two ManhattanGMAT tests or two 800score tests.

The GMAT scoring algorithm has almost nothing to do with the absolute number of questions you get right and wrong. More important is the difficulty of the question and your performance on the questions that preceded it.

Remember the following:

The GMAT is designed to target 60% accuracy across the test regardless of ones performance. That is: someone with a score of 400 should expect to have ~22 questions correct and someone with a score of 600 should also expect to have ~22 questions correct.

Both ManhattanGMAT and 800score use an adaptive algorithm similar to the real GMAT to produce predicted results. You should trust the output of the exams and assume that any changes occurred within you, not within different practice test score methodologies.

-Tom Rose
www.TheMBAShow.com

Full Disclosure: I work for ManhattanGMAT