Like I have said, it is probably due to the excessive amount of randomness involved in deciding the outcomes of matches.
But, there may be something else involved too. I have noticed that for the Cup, many times a team is given favor in one leg and the other team is given favor for the other leg (irrespective whether the team is playing away or home). So, if this is observed way too much, there is surely something else involved other than randomness unless if the favor given is a 50-50 decision.