(There was a big kerfuffle a while ago when an email of this type got leaked.)
There's another case, though, when the startup has initially raised money at a higher cap than the market will bear. We warn founders about this, but they don't always listen. In that case they give the earlier investors the same lower cap that they negotiate with later investors.
The alternative would be a low initial offer which would essentially keep anyone else from paying more. In essence the YC start-ups are giving the initial investor a money back guarantee on the difference between their offer and the lowest offer.