Everybody suspects (no scientific proof) that it's due to the vast majority of people not buying park hoppers. Since they know that the park is open 3 hours later than normal, they are stuck with staying at that park the whole day.
Same thing for morning EMH. For those staying on site, they get stuck at the park that they started with. Then the normal crowd comes in, and thus it looks more crowded.
That's 1 theory, anyway....