Remind me why the Doctor didn't need to ask Donna what year she'd come from, again?

I noticed that too, and was absolutely certain that it (and the fact that Donna didn't know about the Sycorax invasion and the battle of Canary Wharf) was going to turn out to be a plot point - that the Doctor had arrived at Christmas 2004 or earlier, and that the episode was going to end with him getting a glimpse of Rose before he'd met her.