I've always wondered about this and would appreciate it if someone could explain.
I thought it was a holdover from the newsstand days. They would put a later cover date on the books so that they could stay on the stands longer and still look 'current'. It gave them a longer shelf life.
I would guess they keep it because it is tradition, even though they no longer need to worry about the actual news stands.
Makes sense. Thanks, guys!!!
Rob is right; in the old days it was common to date them a few months ahead in hopes they would actually be kept on the racks for that long.
If anybody ever needs such info, Mike's Amazing World of Comics is an exhaustive database of comic publication info that allows users to search for comics by both cover date and actual publication date. It's a wonderful resource. =)
What an amazing resource! Thank you sir!