I have a little hole-in-the-wall anime store that does that with a lot of the English dubs. What they told me is that the release date, at least concerning DVDs (not sure how much of this crosses over into the manga, though I'm sure must be somewhat similar) is the date they expect it to arrive at, say somewhere like Suncoast; this is after going through the warehouse. Somewhere like this little place, that doesn't have a warehouse, gets it two weeks early, and justs throws it out on the shelves, regardless of a release date. Obviously, Borders have warehouses, but it's possible they got their shipment earlier than expected...? Maybe?