Working in retail, there's a lot of miscommunication that happens once you market something for a specific timeframe, and then due to market shifts or inventory issues (both positive and negative), the release date changes. It causes confusion and delays, whether in product being release prior to media blitz campaigns, or vice versa, where the media comes out, and then has to be re-released due to a lag time in release date. This happens constantly in the entertainment industry, especially pre-internet music. CDs would have a release date (always a Tuesday), with a ship date and a receive date. Once some market factor causes the ship date to change (which is simple, since the public does not have access to that generally, it's simply communicated via email and updated on networks linking inventory systems), the previous marketing (print ads, photos, audio/visual advertising, etc.) are all rendered incorrect. Thus, you either have the product before people think you should, or after it says it will be there, and lots of confused unhappy customers.
It's a tangent, but that's part of the reason (with cost still being the overriding factor) that has destroyed the music industry.
Long story short, the concepts apply similarly for LEGO. You could be seeing sets before pre-determined release dates due to overproduction, or it could be (like others have said) pre-existing contractual obligations with third-party retailers (i.e. Toys R Us).