I'm assuming they're going to have that be explained in part two. It would fit in with the way information is presented in general--while in the book Hermione pretty much explains almost everything before they even go anywhere, in the movie she's figuring things out as they go and doesn't always have all the answers. (Quite a reversal from the way the films usually write her.) It's also possible Peter Pettigrew's demise may have been moved somewhere into part two as well, although I'm not sure exactly where that would fit in.

The only thing that was jarring to me was that the shard of Sirius' mirror, which had been cut from the OotP movie, is re-introduced without any explanation at all. The role it played in their escape from the Malfoy's house makes this a fairly large oversight; but again I guess they'll say something about it in the second part.