As far as I know, Variant Human is not exactly a subrace
That was my understanding too. It might from a ruleset perspective function like a subrace and that would probably be the easiest way to implement it technically in game, but as far as I’m aware variant humans aren’t considered in universe to be either mutants or a separate genetic group (or whatever passes for that in the FR!). I’m not sure if there’s actually any in-universe explanation of why some humans might be less jacks of all trades and instead have specialist skills in some areas, though it wouldn’t seem too hard to explain in terms of natural variations within the race. Possibly one of the folks here who know their FR lore can expand on that!
There is no lore regarding Variant Humans.
Variant Human isn't a sub-race it is simply an an optional rule in the PHB that gives you a different way to create a Human character.
Although it's used ubiquitously in most D&D sessions it is a variant character creation rule that requires DM permission to use.
The text from the PHB:
"If your campaign uses the optional feat rules from Chapter 5 your Dungeon Master might allow these variant traits, all of which replace the Human's ability score increase trait"
