We do track results across all of our students and the standard deviation from last practice test to real test score is 50 points.
Obviously some people do fall outside of that range, by definition, but you would be doing yourself a disservice if you simply blamed it on the practice test. You need to figure out why your score dropped so significantly so that you can take concrete steps to prevent it from happening again next time - otherwise, you can't go in with confidence knowing that you have fixed all of the problems.
Read this article and do the analysis described:
http://www.beatthegmat.com/a/2009/10/26 ... went-wrongIf you would like, you can share your analysis here and we'll tell you what we think.
Note: you don't mention fully how you were taking our practice tests (with the essays, for example?) nor how you handled repeats (answer them more quickly or even immediately, thereby giving yourself an artificial time advantage? answering correctly even when you got the question wrong the first time, thereby giving yourself an artificial scoring advantage?). Those things can artificially inflate your score quite a bit, depending upon the variables.