It is actually pretty standard practice. Last week, I was shopping for a hotel (somewhere other than WDW) for an upcoming vacation, and clicked on promotion after promotion, only to find that the specific week I was looking for (January 29 - February 5, 2011) was blacked-out, at a number of places (but not others). In some cases, I was able to get the promotion if I moved a week or two earlier or later. There is no rhyme or reason to it except as a reflection of the load-levels, both already-booked and projected, at that particular venue.