I often experienced this in my studies. The reason for this, was I put too much consideration into the practice test results. For example, I had a week were I score 99% in quant and 70% in verbal, and the next week i made a complete switch. I think there are three reasons for this:
1. Lack of accuracy of the testing algorhythm.It's impossible for these companies to mimic the exact algo from the real GMAT. So sometimes you will get some strange results.
2. I usually concentrated on the section on didn't do as well onAfter scoring in the 99% for the Quant, I felt like it was time to really step up my game in verbal. I spent the whole next week studying only verbal. It would make sense that after a week of only verbal that I would then decrease overall in quant and increase in verbal on my next practice test.
3. Just plain luckSome days you are on, and others you are not. Sometimes you are 100% in it in quant and somtimes you aren't (same for verbal). It just is what it is, and I would put too much stock into it.
Overall practice tests are just that;
practice. So just know that you havn't mastered either section yet, and continue to study your hardest in both sections so you can put that Q51 and V43 together on the same day. (thats a 750 I believe!) Good luck!