I think part of the confusion here is the MSAR uses percentile, whereas MCAT scores are reported as

percentile ranks.

The formula for percentile ranks is:

FORMULA
Where cl is the count of scores less than the score of interest, fi is the frequency of scores of interest, and N is the number of examinees. This formula implies that a 100th percentile rank is impossible. For example, out of 100 students, if 1 student scores a perfect 100, then the percentile rank would be (99 + 0.5*1)/100 = 99.5%

The percentile rank is the percentage of scores below or equal to a given score, as hurtem was alluding to, but it takes the midpoint of the frequency of the scores at the given score. This is demonstrable with the following image from

Definition of Percentile Rank
IMAGE
Notice that the percentile rank for "B" is 77 and not 92, because it takes the midpoint of the B scores. So a true 100th percentile rank should still be impossible, but my guess is that they still round these scores up as efle stated.

edit: the pics weren't showing up so I put links to them