If I were you, I'd probably call them up and ask them. If I had to guess, I'd say it was an outlet preference, maybe trying to ship a few at once and make a profit off what they save with combined shipping.
That's what I'm doing with this thread, killing time
I ordered the 24-inch, 2.8 with 750gb. Some folks might say that with that price I could have gone for the Pro, but adding the display and the extra hard drive goes way beyond my budget and I don't thinking that I need such a powerful machine.
EDIT: Forgot to add that I will hook up the imac with a hifi Marantz amplifier and with my tv. I hope it will result in a kick ass media and working center.