Because the sales to book stores are returnable, book stores often get a lower discount than the non-returnable hobby sales. I.e., WotC makes more money per copy (but incurs the risk of returns).
Interestingly, this means that the more books sell via the book trade (say, because the books are not yet available in the hobby trade), the more cash from those sales goes into WotC's pocket, and the less returns risk they face.
I've heard that many stores with Ingram accounts ordered the books as booksellers (getting a lower discount on those copies), in order to have them on sale early. Ingram apparently made no effort to enforce or even to inform accounts of the official street date for the books.
The tragedy of all this is that a lot of stores were planning big events to coincide with the release of the new edition. Those events are now shafted, as mass market and book trade competitors have the books on sale early. There's no point in having a midnight Harry Potter party -- a week after your competitors sold the books to your customers who are the most enthusiastic fans. (And the success of the worldwide street date of the recent Harry Potter book demonstrates that the book trade and mass market CAN enforce street dates, if their feet are held to the fire.)
However it came about, this is a massive betrayal of the independent game retailer (and the game distributors as well). As a d20 publisher I am disappointed because I see retailers not making profits that they could have to reinvest in improving their stores (i.e., stocking more of my products! as well as merchandising, better fixtures, etc.). I am also concerned that retailers will be investing in more inventory than is really appropriate, given the botched release date. I hope that distributors are giving retailers a chance to reduce their orders -- and, in turn, that WotC is allowing the distributors to pare back their orders, if necessary, due to the sales that went early and unfairly through other channels.
