Early vs late
Nits by themselves are only seen after new lice
have become sexually mature.
That takes ~10 days, days that you can be infested with juvenile lice before they start laying eggs. Nits are laid at skin level, so the distance from the scalp imply s the duration of infestation. After treatment, a dead egg casing may remain. A good soaking with vinegar often loosens the nits & may kill lice.