Well technically, if the park is usually only open to 12pm, then closing at 11pm gives you 11extra hours. But that's just me splitting hairs and being annoying.
You should check out this site - it describes the reason for closing 1hr early Sunday thru Thursday from late July through August:
http://www.miceage.com/allutz/al062006a.htm
I've linked directly to the article - the section that addresses closing times is on the 2nd page about 1/4 way down.
-kali