Neither. there are far to many varibles that would determine how much money a guys stuff is worth. Sure, level and class can help determine that, but most of the time its social class.
I think its for balance reason, if PC get treasures from NPC with PC wealth it will increase the gear value of the PC, and giving them a big advantage.
If the D&D wealth system had been balanced so that the gp by level was multiplied by 13 at each level this kind of problem would be resolved but then you'd have high level PC with enough wealth to buy several kingdom.s
PS: in fact it should be a formula close to:
(base gp)*(13.33*0.80*0.80)^level