We do not systematically over or under-inflate scores, but any CAT (ours, GMATPrep, the real thing) is nowhere near as predictable as most people assume.
The standard deviation on the official test is about 30 points - yes, even though they give scores in increments of 10 points, the SD is 30 points. This means: take your score and add and subtract 30 points. Your score is not considered statistically different than others in that range of scores. (Eg, for a 680, your "true ability" is considered to be in the range 650 to 710, and you would have a 2/3 chance of scoring in that range if you took the test again.)
The SD on ours is about 50 points. GMATPrep's SD hasn't been published, but I assume it is better than ours and not as good as the real thing's SD, so call it 40 points.
A better way to determine whether to take it again is to try to figure out what went wrong in verbal on test day. Was your timing okay or did you run out of time towards the end and have to rush? What do you remember of individual questions? Did you get more than usual in your particular areas of weakness? Were there any that really threw you and caused your anxiety level to spike? Were you more tired than usual by the time you got to the verbal section? When you took practice tests, did you make sure to do the essays fully, every time, to practice what the full scenario would be on test day? Etc.
Please feel free to reply with answers to any of the above questions and we'll help you figure out what went wrong. Also, if you took our course, you have the option to have a free Post Exam Assessment phone call, which is a debrief session to figure out what to do if you decide to take it again. Please contact the office (800.576.GMAT or
studentservices@manhattangmat.com) if you are a former student and would like to do this.