I wrote in a different stand alone comment but I think this has to do with button down being just unreliable for mechanical buttons. The system always has to do some debouncing, so most GUI systems only listens to button up, and only in games the down event is used with extra caution.
Debouncing was literally several classes in my college years, as I recall. Can definitely be more complicated, but I find the anecdote of a team trying to insist that it isn't a better experience mind blowing.
Granted, just the other day, I found that in GMail key repeat events are slow as heck for some reason. Made holding backspace to delete a sentence annoying.
