My DS is 19m, so I have no idea, but this thread reminds me of a conversation I had with my MIL talking about her three boys growing up.
The middle one did everything late, but would do things well quickly after he started. She puts this down to temperament and not wanting to try things and fail at them. So he waited till he could do something confidently before getting stuck in. My DH (youngest), by comparison, just threw himself into everything. DH did achieve more academically than his brother, and my MIL wonders if a feedback loop of lower expectations affected her middle son, who is a really clever and lovely guy but still with low self-esteem. There's a similar dynamic with me (younger child) and my sister (older child).
And this is turn reminds me of something I saw on tele about make vs female brains and experiments which show mothers playing with baby boys expect them to be able to handle more physically than mothers of baby boys, so while physical capability is the same, the boys are consistently challenged more.
Long-winded, but what I'm getting at is that a) lots of variation may be due to temperament / learning style rather than ability and b) in the vast majority if normally developing kids we almost certainly, if unwittingly, reinforce differences by early labelling / measuring against peers. Not that I think we can avoid it, just that it's probably always a factor.