The PR tests are adaptive like the real GMAT and have a very similar scoring algorithm, but I'm not sure about the Kaplan tests. That could explain some of the difference. Also, I think both PR and Kaplan say if you apply their techniques to other simulated tests, they won't work as well (I used PR's strategies and they worked just fine on the real GMAT, though). Have you take a Powerprep/GMATPrep test yet? They're adaptive too, so that should give you an idea of where you stand.
If you've only taken a handful of tests, though, and haven't done just practice questions in between, your scores can fluctuate a lot, so don't worry about early scores. Do practice questions on weekdays and a full practice test each weekend, and you should see an upward or at least more consistent progression! If you're doing that already, the one test score could just be a fluke. Hard to say!