At first I thought it was because development moved faster than expected, but that's not the kind of thing you leak. If you're ahead of schedule, you launch. You don't predict a launch. Also, who has ever been ahead of schedule?
No, I think the iPad subs are a lot lower than expected and/or cost is a lot higher than expected. At least, lower than Murdoch expected; any one who used it probably would not be surprised.
via All Things D