WDW had to give a LOT of comp tickets in those first few days because they overestimated what they/the ride could handle. I think it is a really rational business decision to aim low in the first week or two. They will save themselves a lot of money on comps, and it's not like they will close the ride at 4 pm if all the boarding groups have been called. They still have the backup boarding groups, and then those people will be really happy.