If I pull a FP with a 10:00-11:00am return time, I take that slot out of the system. I then don't return until 5:00-6:00pm, and use my 10-11 FP, that's using it late, obviously.
If I pull a FP with a 10:00-11:00am return time, and then pull a second FP with a 5:00-6:00pm return time, and only use the 5:00pm-6:00pm FP on time, then I'm not using a late FP obviously.
The difference with FP+, is if you want the later time, you'll need to adjust your experience - BEFORE the existing experience time arrives (or ends, unclear). If it expires, it is lost and can't be rescheduled. At least now, according to the T&C. So, it's not being able to show up late - it is being able to reschedule - and in theory, possibly put a slot back into the system if it's not too close to be able to do it effectively (imagine the confusion if you walked up at 9:55am to get a FP showing 2:00-3:00pm, but then suddenly one for 10:00-11:00am pops out - but only one, and you were getting 4...)