Hi 600orBust,
The score calculator is calibrated to our older tests; we've gone through several revisions over the years, and our newest tests are quite accurate. Can you specify which test, exactly, you're taking? I would be happy to give you a more specific answer once I have that information.
That being said, even if you got a 550 on our most recent, and accurate, algorithm, that still shouldn't be a cause for concern. There a a huge number of other factors that can influence a practice test, from distracting phone calls to exhaustion from studying to a flaky internet connect. When evaluating a practice test, it's vital to analyze not only your performance on the individual problems but also your focus and energy levels throughout. A drop in a score isn't a limitation, but rather a valuable learning opportunity--if something went wrong on our test, then you can make sure that it doesn't happen again on Test Day!
Please let me know if I can answer any other questions. And in the mean time, congratulations on choosing Kaplan to help you with the GMAT, and best of luck on your studies.