Nope, and this answers your initial question exactly:
Is the exam graded on a curve?
Examinees often ask if earning a high score or higher percentile is easier or harder at different times of the testing year. They ask whether they have a better chance of earning a higher score in April or in August, for example. The question is based on an assumption that the exam is scored on a curve, and that a final score is dependent on how an individual performed in comparison to other examinees from the same test day or same time of year.
While there may be small differences in the MCAT exam you took compared to another examinee, the scoring process accounts for these differences so that an 8 earned on physical sciences on one exam means the same thing as an 8 earned on any other exam. The percentile provided on your score report simply indicates what percentage of examinees from the previous testing year scored the same as you did on the MCAT exam.
How you score on the MCAT exam, therefore, is not reflective of the particular exam you tookincluding the time of day, the test date, or the time of yearsince any difference in difficulty level is accounted for when calculating your scale scores (see above for information about scaling).
Source:
http://www.aamc.org/students/mcat/preparing/understandingscores.htm