Because next day deliveries are done through couriers who guarantee the delivery date, this means Amazon don't even try to ship those ones until the last minute, because they can be certain it will reach the customer on release day either way.
Royal Mail on the other hand is a lot less reliable, so Amazon ship those a couple of days earlier to stick to their release day promise, which results in some people getting them early.
Ah I see. Didn't know about that. Suppose at this point the difference between Monday and Tuesday isn't too big of a deal.