For what it's worth, I took our free practice CAT and scored 10 points less than I did on the actual GMAT - so, I think they're pretty accurate. And we definitely don't purposefully make the tests easy! That might make students feel good during the course, but we wouldn't have very many satisfied customers come test day! :)
The fact of the matter is that no practice test can be perfectly accurate. The real GMAT has a margin of error of +/- 30 points, so practice exams - which are simply guesses as to how the real GMAT functions - are going to have even larger margins of error. I encourage my students to think of their practice tests as a 100 point range. So, if you scored a 550, your "real" score is somewhere between 500 and 600. I know that's a huge range, but it's really the best that anyone outside GMAC can do.
So, if you've scored all over the place between 590 and 670, you're within that 100 point range, and your true score is probably somewhere in between.
Jim S. - Veritas Prep GMAT Instructor (Los Angeles, CA)