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15 was chosen back when they believe that it was the average of 4d6 drop low. Mark Seifter (back when he was Rogue Eidolon) showed that 4d6 drop low is actually on average closer to 21 points than 15.
Seifter's calculation was predicated on the observation that "when actually rolling for stats, basically everybody rejects low rolls that would result in a hopeless character" (like a 6,7,10,12,9,11 array).
Sort of. I was analyzing the original math of 3.0/3.5 D&D 25 Point Buy (the equivalent to Pathfinder 15 Point Buy), where there actually were specific rules for a "hopeless character" that entitled you to a reroll. At the time, after someone was worried that throwing out the hopeless characters was the main cause of the finding, I also calculated without removing hopeless characters and it was a little lower, if I recall a little under 1 point, but still way higher than 25/15; there just weren't that many characters that were hopeless and they didn't affect the numbers that much on the low end as the characters with high stats did on the high end.