Not weird at all. You posted without the details expanded, so we can't see the fare bucket. But, chances are that Y or something close to it is all that is left and the seat in F is either an upgrade or cheap F.
The difference is more than likely that the Y seat is fully flexible while the F seat is highly restricted.
But, if you know that you are not going to need to change, I would certainly book the F seat. Whether the entire thing is a good deal is not material if it's a trip you must take and the customer, e.g. your employer doesn't worry about money.