According to my LSAT/LSDAS booklet the last test's total (JUNE) number of testers was approximately 23,400 and in order to get the average of what the avg was approximately was to put into account the largest numbers of the same scores recieved so:from 144-150 had the most students score in this range so:
144 + 150 (high and low) = 294
and 294 / 2 (to get the mean or average) = 147!
Based on this information compared to where you heard that 150 put you in the 37% is luckily mistaken (these numbers came from the actual registration booklet of the 2003-2004 academic year meaning the last given LSAT before publication was JUNE. 150 actually put you in roughly the 51-52 percentile range. The 37% would be in the low to mid 130's.
long story short:
150 = 151-152ish percentile over the rest of test takers
132-133ish = 37 percentile over the rest of test takers.
Hope this sheds some light...