I don't know the answer, and even after spending like ten minutes looking for it, I still can't find it. I'm pretty sure it comes from LSAC/the ABA, though. So, my guess is that it's the GPA you were accepted with, because LSAC would have that, while the schools would have to self-report final GPAs, and they would not have undergone the LSAC standardization process (at least in the case of people coming straight from undergrad). So I am fairly confident (though, again, just guessing) that it's your LSAC GPA.
Point two in favor of this theory: Let's suppose that most schools would like their GPA quartiles to be as high as possible. If the "official" GPA were based on final GPAs rather than LSAC GPAs, then it would be much more difficult for AdComms to predict their school's numbers based on the GPAs they accepted. If I were a school operating under that policy, I don't think I'd accept anyone from undergrad until after fall grades came out, because they would still have 25% of the UGPA to go, and anything could happen. The risk would be much less if the GPA were "locked in" when the student got accepted. Since the law schools themselves set the standards for this data, I'm sure they'd opt to be risk-averse.