Originally Posted by
Moi
"Correlation does not imply causation"
The fact that you have won more matches without this bonus than when you have this bonus doesn't mean there's a correlation, as there are hundreds of others variations and variables involved that you are not adding to the equation (opponents' quality, own players condition and morale, tactics, if you watched the match or not, individual tactics, supporters, formation, team style ...)
Giving a win bonus gives your players a boost, but does not mean you will automatically win all games. And not giving it doesn't make your team weak and will make them start losing all games. It's just an extra bonus. Chances are that you would've lost all the matches you lost with the win bonus, maybe by even more goals.