Band/record label websites can be known for jumping the gun, I guess. When I talked to the guys from Dark Tranquility in early 2003, they were absolutely convinced that they were booked for WOA 2003, and so was their record label. Shortly thereafter, the Wacken organizers quickly shot that one down. Things change.
Sites like Blabbermouth are rumor mills by definition. The Wacken website has the reputation of the festival on the line - if they announce bands before the ink is dry on the contracts confirming their appearance, they risk suffering cancellations, and that just plain looks bad. (and very Jack Koshick/Milwaukee Metalfestish, if ya dig)