Sure in the book Dumbles was dodgy about what was up with his hand but I think he did eventually explain it (if not the whole story) whereas in the movie...he really never did.

No he didn't and it was frustrating. I mean I totally get that they can't put everything from the books into the movies cause they'd just be so insanely long but there are things that don't make sense when certain parts of the story are left out.