Rarely. The reason I think first finishers end matches more often on fast is because CPU will often choose to use an OMG with it's first finisher icon instead of its finisher. I'm not sure why it does that, but my best guess would be because on fast the victim isn't low enough on health for the CPU to consider using a actual finisher - they're in yellow most of the time. Of course the power of an OMG is every bit as strong as finishers, so when they hit one they're in prime position to use their second finisher icon as an actual finisher and win the match. On normal and hard, a wrestler is often worn down enough by the time a finisher icon drops that the CPU will think it can win the match with that one move, and it usually doesn't.
This is assuming Resilliency is not in the equation. If it is, that "free" kickout will often mean somebody's kicking out of a finisher regardless of how much damage has been done to them. Maybe you'll get lucky and see it burnt on a signature move (which can also end matches if someone's worn down), but it'll usually be used on a finisher.
That's my take on it - it's the only way I can justify why Fast sees matches end on first finishers more often.