Well I think every HP fan, who has read PoA, was kinda huh about that...

I know I was and still am...

I dont think there is any explanation exept that this is afterall the film not the book and some things are changed - and some without a reason...Im sure there are many other examples of the fact that the filmmakers change things ...

I dont know why they did it - I dont see the point, because they could have just done it like its done in the book but well I dunno...