I don't think they altered the pic. The lighting is different because the big memorial monument where the ceremony is at is probably blocking the sun. The man's faces who is waiting to hand the wreath to whoever is coming to get it behind Kate doesn't have the sun in his face or shadows showing either. And why should they make changes to that pic? It shows nothing of interest or worth such work.
According to The Telegraph, the Cambridges were not supposed to attend the ceremony. If that's correct, then why did they? When royals go to an event, it's always planned. They don't do "spur of the moment" things. So here's my jaded take on what happened: Waitey decided to bring her whole family along to the church service as a PR stunt (witness, her family members grinning at the photographer, despite the fact that there is no reason for them to), then decided she and Wm. would attend the ceremony with the Queen. But instead of thinking about the 58,000 dead soldiers that are being commemorated, Waitey stares right into the camera without one ounce of concern for the occasion. Maybe that's why the other people at the ceremony are not smiling -- they are thinking about the 58,000 soldiers killed. At least Wm. had the decency to have a serious look on his face during the ceremony; Waitey couldn't even muster that.