If I switch the last match-up to barcelona beating man utd I get negative numbers:
man utd :: -0.363
psg :: -0.375
ac milan :: -0.394
barcelona :: -0.439
matches = [('barcelona', 'man utd'), ('man utd', 'ac milan'), ('barcelona', 'ac milan'), ('ac milan', 'psg'), ('psg', 'man utd'), ('barcelona', 'psg'), ('barcelona', 'man utd')]