At this point, I wouldn't be worried if I were you. Different companies have different scoring scales, and it's very typical to see a score drop when switching to a new company's tests. One deviant score certainly isn't enough to warrant panic, especially considering that score improvement is rarely linear, even on exams from the same company. 509-511 on Kaplan exams is also a very solid place to be considering that you're still 2 months out from your test date.
That being said, there are students who perform significantly better on one particular company's exams than on AAMC material or other tests. Luckily, there are a bunch of things you can do to make sure you have as realistic an idea of where you are as possible:
- Try a variety of practice exams. One TPR exam isn't enough information - maybe you just had a bad day, or maybe that particular test happened to ask about all of your weak topics. You have plenty of time, and many companies offer free tests or other resources.
- Supplement with AAMC practice (especially the Section Bank). Try to alternate AAMC and non-AAMC material as often as possible, and review everything about the AAMC practice. Getting accustomed to AAMC wording and style is extremely valuable, and if you do well on these passages, it'll boost your confidence that you are where you think you are in terms of score.
- Isolate exactly why your score dropped on the TPR test. Did you do anything differently from your typical approach to a Kaplan test? Did wording/passage style/etc. throw you off? Or did you approach the test normally, but happened to run into topics that gave you trouble? This can be a great way to get to know yourself anyway.
If you do all of these things, you should be in a good place. And the short answer to your question is yes - scored practice tests should be a good indicator of your score on the real exam, but individual students and tests definitely vary. Luckily, now the AAMC has two official scored exams that you can use to predict your score closer to your test date.
Good luck 🙂