Hmm. That is puzzling. I can think of a few potential causes.
I agree that the official practice tests are the best ones...but I'd also like to know what's causing the varied performance, in case it's something that could crop up on the real thing. And presumably you want to know that, too. :)
First, were you taking all of the tests under 100% official conditions, including essay and IR, as well as the two 8-minute breaks? Deviating from official conditions - taking a longer break, skipping a section - can result in inflated test scores on Q and V (since you are not using up as much mental energy as you will have to use on the real test).
The second GMATPrep test, for example, uses the same IR questions as the first test, so it's not a good test of the mental stamina you'll need. (That doesn't explain things though, since your first GMATPrep was the same very high score.)
By the same token, were there any reasons why you would have been more mentally fatigued than usual, especially when starting the 3rd MGMAT CAT? (the only one that technically represented a drop in score...)
Next, what timing issues do you have? Notice that I don't ask whether you have timing issues - that's because everybody has at least some minor issues. (Note: you can finish a section on time and still have timing issues.)
Serious timing issues can cause score fluctuations. If so, they can crop up on the real test too.
Use this article to analyze your MGMAT CATs to diagnose your timing issues:
http://tinyurl.com/analyzeyourcatsYou don't have the same data for the GMATPrep tests, so you'll have to analyze based on what you remember. Compare the above analysis of MGMAT to your best-guess memories of GMATPrep.
Let me know what you think. Also, if you do discover timing issues, you can start to address them using the following articles:
overall mindset:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/timing:
http://www.manhattangmat.com/blog/index ... -to-do-it/http://www.manhattangmat.com/blog/index ... nt-part-1/