Lies that make no sense

So I got a book in the mail today from Bookmooch, well not from Bookmooch, but through the service. Anyway, the person sending the book marked "sent" way back in December. However, the post mark on the package was more than a month after that. Why mark the thing "sent" before it is sent? I'm not really complaining, I'm just curious.
Bookmooch, for those who aren't in the know, is a site where you can trade books through the mail. Nifty.

Here's my current inventory. Not much, I know, but I've got a few I'll be throwing on there soonish.