Not only Knewton CATs, but all the available CATs in the market have their share of problems, specially in the scoring methodology. They will show you a score which will be different from the GMATprep software and also they will markedly differ among each other. My suggestion, take as many practice CATs as you want, but for practice sake only, do not trust them for score assessment. The GMATprep is the only test you should trust to assess your scoring level.
Also, you may take the same GMATprep CAT couple of times. You will almost get 80-85% different questions each time. This way, you will be exposed to the maximum number of real test-like questions.