Yes, there is, because plans can change. It doesn't happen often with Wizards of the Coast, but it has happened - they've had to push books back from their original release date, like Ghostwalk; they've quietly cancelled some announced books, like d20 Spectaculars; they've even put together at least one book in less than their usual leisurely (for this industry) time frame because of a hole in their schedule, like Dragon Magic.
Look at smaller companies in the industry (they're all much smaller): there are dozens of companies which never release books they've announced, push books further and further back on the schedule, rush books out the door faster than they should have been, et cetera.