I think demand was way way more than anyone expected.
Analyst predictions, which probably wern't too far from Apple's predictions seemed to be around 600,000 for the weekend and that was considered optomistic. Reports are estimated now to be around 1,000,000 sold on that weekend.
If things went according to the estimates, they would have sold 600,00 first weekend and still had stock for the follwoing week with shipments slowly trickling in.
Sometimes you just can't plan for everything.
So are they basing this on the first ipad then? They are obviously not basing it on the Iphone 4 figures. But then again I'm not sure how many were sold at launch, its got to be quite a few though.