I think it's because he won't have as much to do and I think he did very well in the movies where it was really centred on him like POA and OOTP. He probably knows he performs best when he can be morbid. He'll have his chance again in Deathly Hallows although strangely enough, I found Deathly Hallows Harry Potter more upbeat than the OOTP HP. However, my all time favourite Harry was the one in HBP so I hope he can pull it off adequately although he doesn't particularly care for the lightness.