I got to thinking about it some more, humorously imagining a cat having to chew its fur down for a trim, when I think I hit on what's going on there. I'd bet a bright shiny quarter that if you compare two cats who are otherwise identical, other than one being long-hair and one being short-hair, you'd find that the short-haired cat sheds more individual hairs.
Let's say you have a cat with 1" long hair, and a cat with 1/2" long hair. I'm betting that both of them shed the same volume/mass of fur over a given period of time, but the short-haired cat sheds twice as many individual hairs. It's not that the hair somehow knows when to stop growing, but instead there's an expiration date at the root. The longer the expiration duration, the longer the hair can grow. The shorter the expiration duration, the shorter the hair typically is.
I've no proof, but this seems a pretty likely theory.