Standard deviation from last practice to real test is about 50 points. As it happens, standard deviation between practice tests is also about 50 points - different data set, of course, and just happens to give similar results.
Are you taking the tests under 100% official conditions? That's very important. Also, were those three tests taken under relatively consistent conditions, or did you do things like skip the essays on some, take longer breaks on some, use the pause button, etc? Those things can all make a difference to the validity of your score.
If you haven't already been doing so, take all future tests under 100% official conditions. The more you deviate from official conditions, the better the chances that your practice scores are not all that great as an indicator of your current ability.
Next, you mention making mistakes. That's part of life. :) We do want to try to minimize them as much as possible, of course. This article can give you some ideas about how to do that:
http://www.manhattangmat.com/articles/error-log.cfmAlso, are you having any timing problems? A lot of times, when people find their scores sort of stagnating even though they've been studying a lot, timing is one of the reasons.
(Oh, and how far apart did you take the tests? If you took them within a few days or a week of each other, then you really shouldn't expect much difference in the score. It typically takes several weeks at the very least before you can see a noticeable difference.)
Back to timing. Use this article to analyze your last test or two:
http://www.beatthegmat.com/mba/2011/02/ ... sts-part-1Then come back and let us know what you discover.