if they're waiting for stock to come in (backordered), they'll typically pad the result a bit and tell you a later date than what they expect, so if it shows up when they think it will you think it's early and are extra happy. And if it does show up late, you still think it's on time, and are happy. It's not till it's REALLY late that you start to get un-happy.
Plus it tends to whip up some buzz, since "supplies are slipping to mid June"