I have to believe that if you're suddenly unable to cancel, it's because your phone has been allocated to you and is somewhere in the boxing/loading/transport process. Whether it has an actual impact on the date of receipt, okay, maybe not. But I don't think Preparing to Ship is assigned randomly.
Yeah I think they just pull them out by time. And I mean of course that doesn't mean the 3:00 who changes first will get theirs before a 3:07, just so long as they're changed the same day it shouldn't matter.