"Normal people" NPCs get 15 points, and have only NPC classes.
(11, 11, 11, 10, 10, 10 or 13, 12, 11, 10, 9, 8 or something similar)
"Exceptional people" NPCs get 20 points, and have only NPC classes. This represents natural talent above the normal people.
"Trained professionals" NPCs get 25 points, and have PC classes. This represents some natural talent and training. (Even the great archmage of the kingdom and the captain of the king's guard is built on 25 point buy, plus the normal bonuses for every 4 levels.)
Only PCs get 28 points. This represents natural talent and training. PCs are special.
In theory, if an NPC with an NPC class gets training in a PC class, their stats increase. The physical and mental training for a PC class builds on the character's natural talents.
Quasqueton