Oski, i think they're based on 1 match not the whole tie.
I was semi-confident then i read these:
Manchester United are unbeaten in the Champions League, home or away, for an incredible two years. The last team to beat them were AC Milan in the semi-final second leg two years ago. The Rossoneri thumped the Red Devils 3-0, with goals from Kaka, Clarence Seedorf and Alberto Gilardino.
Arsenal have a poor record of comebacks in Europe. During their history, of the eight times they have lost in the first leg of a knockout game, only once have they then gone through. This was against Hajduk Split, 30 years ago.
United will go through tonight if they keep a clean sheet, and they have not conceded a goal away in a knockout tie for four matches. Roma, Barcelona, Inter and Porto all failed to score at home.