The questions in the GMATPrep test software are retired questions that were once part of the official test. So, these questions have been asked many times (in real testing situations), and the test-makers have tons of data on them. They use this information to calculate your scores (and to determine which questions to throw at you during the test).

Yes, it seems odd that 78 and 67 percentiles would yield a 81 percentile score, but many students aren't very strong in both sections. Some get huge quant scores and really bad verbal scores (and vice versa). So, getting solid scores in both sections must be more exceptional.

Cheers,
Brent