Oh, wait, 15 point buy would get you the 3d6 averages. 25 point buy is roughly equivalent to 4d6b3. Right. Okay, so PCs get slightly above-average stats at the beginning because of the assumption that most of the people who go adventuring are the unusual ones. That accounts for all the real superiority PCs have to NPCs at level 1, besides a couple of small class abilities.
This doesn't argue my point that NPCs are held to the same rules as PCs. If a PC should be able to lift more, give them higher Str scores, don't change the rules that apply to normal people too.