How about taking a look at the PL rivalry statistics as well 
(33303.1)? I just realized the problem is not limited to PL games (see my second example).
I have played a total of 6 PL games against Olimpia Levico Terme 
(27132) and lost 4 times. Yet my rivalry statistics show 6 wins for me. The second example I can give is Pahalammen Pomppu 
(58913) whom I have faced in the league as well as Private league. I have also played them in the Cup once.
My games against Pahalammen Pomppu 
(58913):
PL: 4 games, 
1 won, shows 5 won (yes, one more than played)
League: 3 games, 1 won, shows 1 won (so, this is correct)
Tournament: 1 game, 
1 won, shows no wins for either side (counted as a PL win)
Total: 8 games, 3 won, shows 6 won
(Yeah, I know I need to start winning more games against Pomppu...)