I think I see. This is because the point buy value scale is not symmetrical around the Classic (3d6) method's average of 10.5.
We can make these 3 versions of a big, strong commoner.
(13, 10, 12, 9, 8, 11) = 15 points.
(11, 10, 11, 10, 10, 11) = 15 points.
(15, 8, 13, 8, 8, 9) = 15 points...