These are absolutely adaptive - they're built on the same algorithmic principle as the real test.
It's very easy to have a lot of 700+ questions and yet still score below 700. The GMAT operates on a "where you end is what you get" principle. The test is not scored based on an average taken only after the test is over (the way paper tests are).
So, you can be doing well (and earning hard questions), but if your performance drops by the end of the test then... where you end is what you get.
Also, the algorithm is allowed to select questions in a decently wide swath. It isn't the case that, if your currently calculated score is 660, the test will give you exactly a 660 question next. The difficulty might vary anywhere from 590-ish to 730-ish depending on various factors.
If you'd like to get some more detailed advice from us re: what to do with your test results, please use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Then come back here and tell us the results of your analysis and what you think you should do based on that analysis. We'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
I also highly recommend reading these two articles:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/