League Random results

[Kenny Jones]

I do want to learn, and it is not a complaint, it is an observation.
I play against a team in the Champions league 18% higher than me and win 7-0, with my players scoring 8s and 9s and a player on fire with a hat trick.
The next league match, I play exactly my same starting XI against a team 10% lower and lose 1-0 with my players ratings dropping to 5s and 6s, including my on fire player.
I've won every association match this season, no matter what team I put out or who I am playing against. I am in Platinum division, so it is not easy!
Why does my team have such different form in the cups and the league and the association's?
How can the same team be good in 3 competitions but not in the league?