Looking at Common Data Sets and college profiles, I see lots of schools with ever-higher percentages of matriculating students who were in their high schools' top 10%. Some colleges now mention how many students were in the top 5%. At the same time, there are small, highly-regarded high schools that regularly place far more than 10% of their graduating seniors into tippy-top universities and colleges. Some top colleges also recompute applicant's GPAs using the college's criteria. Perhaps they only use academic core courses, or they remove all AP/IB/Honors weighting, or they don't account for plusses and minuses. Yet class rank was set using the GPA calculated by the high school. If the high school used the college's scheme, maybe a kid who fell outside the top 10% would move up in the ranking, or a kid who ranked high would see class rank plummet. For that matter, given all the different ways that high schools compute GPA, "top 10%" could mean very different things in different districts. The "most rigorous courseload" is supposed to help give some context, but that doesn't really say much about how many students in the top decile are doing the most rigorous thing, and how many are taking an easy schedule.
So what does it really mean when a college says that XX% of its freshman class was in the top 10% of their high school graduating classes?
I suppose that the meaning of "GPA" is also squishy, except for a 4.0 (or 0.0, for that matter. Now I'm thinking of John Belushi in Animal House, with pencils hanging out of his nostrils).