I think there are only so many people who really want to ride it each day and all of them are trying at 7am, but then only those that didn't get one at 7am can try at 1pm
So if there are 15,000 people with EP reservations that really want to ride, and only 10,000 slots given out at 7am, well, all 15,000 people trying for those 10,000 slots.
But then if they open up another 8,000 at 1pm, well, there are only 5,000 people left that have an Epcot reservation and really want one so they get one at 1pm and then their are 3,000 slots left that go to people that are open to going if not much effort or after people can park hop