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other NPC doesn't get to level up as they want

Actually, they kinda do. NPCs in 5e doesn't follow in any way character creation. I think you can go as far as finding some statblock where NPCs have access to some spells before their CR should allow so. An orc commoner in "Tales from the yawning portal" have 10 in all stats exactly as a human. Statblocks are designed depending on the narration they want to bring forward.

Again, stats are just a numeric hindrance. Do you want to show that elves are great archers? Give them double range with ranged weapons. Want to show they are agile? Advantage in acrobatic. +2 to Dex adds more problems than what it resolves.