There are 3 reasons to why:
1) They have the ability to rotate mid-field and forwards because there are no 'high-profile' stars and such almost every player can be substituted effectively. This will give Man Utd an element of surprise. However, Barca cannot substitute the core (Xavi, Iniesta) and the attack (Messi, Villa, maybe Pedro) which will give Man Utd a clear idea of what players will be starting that day. Just in case you are wondering, Man Utd can use Scholes/Anderson/Carrick/Fletcher/Park for the core because IMO they are all of the same quality (maybe Park better than the others) and Rooney/Berbatov/Chicha and even Owen. Plus, because Fergie never puts 3 forwards, they have a nice rotation at hand.
2) All the players are in good condition because Schalke was no challenge for them. Unlike Barca, who had to play a dirty Real team from which they are surely going to sustain some kind of injuries. And also, Barca took their focus off, somewhat, from winning CL to beating Real first priority. Not saying they aren't concentrated, but at least somewhat distracted.
3) Wembley. Probably Webb. Man Utd advantage.
- Man Utd ML is currently at around 3.6 and DNB is around 2.7... I will put whatever I can get together for that DNB... worst case scenario IMO is a DRAW, but expect Man Utd to surprise with a 2-1 win.