Query - Performance on MGMAT tests

It's unusual to improve that much in only 15 days, yes. But it's also generally not possible to have a "fluke" high score unless you got unbelievably lucky or somehow gave yourself a major artificial advantage.

For instance, if you did the essays on the 620 test, but skipped them on the 720 test, and gave yourself extra long breaks on the 720 test, then that might explain why one test would be so much higher. In this case, the lower score would likely be closer to your true scoring range, because you would have given yourself advantages that you can't replicate on the real test.

Alternatively, if you messed up the timing on the first test and, say, ran out of time and had a bunch of wrong answers in a row, or had to rush throughout the section and made a bunch of careless mistakes on problems you should've gotten right, that would have lowered your score. If you then didn't have those problems on the 720 test, then the higher score would be closer to your true scoring range.

Or, possibly, all of the above apply, and the "true range" is somewhere in the middle.

Can you give us any insight into what you think happened?

Also, a couple of things. Take all practice tests under 100% official conditions, including essays. Use the below article to analyze your test to see if you have any timing problems (almost everyone does):
https://www.manhattangmat.com/blog/index ... ice-tests/

That will also give you a good handle on your content strengths and weaknesses. Finally, if you do have timing problems, here's how to fix them:
https://www.manhattangmat.com/blog/index ... anagement/

Good luck!