D&D General Should NPCs be built using the same rules as PCs?

[jmartkdr2]

Thing is, is the typical Human in the setting is on a 3-18 bell curve for its stats, then if an Elf gets +2 Dex then that tells me the bell curve range for Elf dexterity runs from 5-20.

Which then tells me an Elf should in theory be able to start with Dex 20 and go from there but cannot possibly start with Dex 4 as the species simply doesn't go that low.

That makes a couple assumptions that while reasonable aren’t necessarily true.

(Also since humans get a +1 floating and a feat their range is 3-20 even under those assumptions)