Sorry for the delayed response - I've been out of town.
So, GMAT scoring is NOT really based on # right vs. # wrong. Yes, of course you want to get more questions right, but the main issue is the difficulty of the questions you get right. Harder questions are worth more points, so you want to be getting difficult questions - even if you get many of them wrong (it's quite possible to get a great score and get a lot of questions wrong, as long as you're into the "upper bin" material). Many students take this to mean that they should only study the most difficult material. This is faulty logic; due to the adaptive nature of the GMAT, you won't even see the tough stuff unless you've mastered the easy/medium stuff. So, at this point, you should be studying medium level difficulty questions and concepts (both math and verbal - it sounds like your skills are about equal in each).
Finally, remember that no practice test is perfect - even the real GMAT has a margin of error of +/- 30 points. Think of your scores as a range, not as an exact value, so you won't be shocked if you go from 600 to 570 to 620 .... those are all within the margin of error and so are essentially the same score.
Jim S. - Veritas Prep GMAT Instructor (Los Angeles, CA)