You look at an item, and it says on the main page "usually dispatched within 1-2 days", let's say, for argument's sake. You place an order, and miraculously availability is now "3-5 weeks".
Or you look at an item, and it says on its page "usually dispatched within 1-2 days". You place an order, and it still looks like 1-2 days. And then you go back a few days later, wondering why it hasn't been dispatched yet, and now it says "3-5 weeks". And you swear a lot, because it was a birthday present for someone whose birthday was, at the time of order, a couple of weeks away.
If they're going to change availability on something already ordered, WHY THE HELL can't they send out an email telling the person who ordered it about this? Do they think we have nothing better to do than to sit there, constantly refreshing the cutesily-named "Where's My Stuff?" page, to see what random time they've picked for delivery this hour?