I bet there aren't THAT many people in all those boarding groups. They likely have a lot of very conservative assumptions in there.
So, if the ride does, miraculously, run well, more slots will open up.
I get that they don't want a 10 hour line like Universal does, but at least people in those lines have.... Hope. Today, if you didn't leave for the park until 8 am, you were pretty much screwed.