Sorry to hear that the actual exam didn't treat you terribly well. A few thoughts...
1) If you've used a lot of Kaplan materials, you'll definitely get used to their writing style, which is different from the style of the actual exam. Inevitably, you'll learn to see Kaplan's traps, and you'll do much better on their tests than on the real thing. This is even worse if you've exclusively used Kaplan materials. I'm not sure that it explains a 130-point drop, but it definitely explains a large chunk of it.
2) Non-official tests (anything besides the GMATPrep) are inherently unreliable, at least to some extent. The Kaplan tests are the work of some very smart people who are trying their best to reverse-engineer the actual GMAT test... but they aren't the actual GMAT. They differ from the actual exam in all sorts of subtle ways, and it simply isn't unusual to see huge discrepancies between Kaplan (or
MGMAT or Veritas or PR) tests and the real thing. Sure, your gap is larger than most, but it isn't completely crazy.
3) Have you taken the GMATPrep tests before? If so, is the 510 close to your previous GMATPrep score(s)? If your GMATPrep and actual GMAT scores are in a similar range, then it suggests that the Kaplan test might be giving you the wrong impression, unfortunately.
4) Did you use the official guides, or just Kaplan materials? I'd argue that the single most important way to improve on the GMAT is to get used to the style of the actual questions. The math on the test isn't terribly advanced, but the questions are brutal because of the way that they're written. If you haven't been exposed to enough actual, retired questions, you probably won't do all that well. Kaplan is a nice supplement to official materials, but a terrible substitute.
5) Maybe nerves played a role during your actual exam? It doesn't take too many careless errors early in a section to send your score plummeting. If you're even the slightest bit jittery or rushing through things a little bit faster than usual, it can have a disproportionate impact on your score.
So depending on how much you've used the official guides, I'm not completely shocked by the score discrepancy. The good news is, if you can improve as much as you have on the Kaplan tests, you can do it on the real thing, too. It's just a question of getting better practice materials in front of you, and making sure that you have a better test day next time.
This is just my opinion, but your situation isn't as strange as it may seem--and it definitely isn't as bad as it may seem. When you finally get that 650, it will feel even sweeter after this experience.
Good luck!