How to be soo good and soooo bad in the same time ?

I know you may have seen this subject many times on this forum and but i never get an answer for it .
How can i be so good in the cup and cl and being soooo bad in the league no matter whan i do :confused: ps : i wasn't lucky in the cup there are teams lv 23-21(stronger than me by +3 qualityor more) and i'm lv 20 and i still beat them .

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Dangg that's weird :D yeah.. How strong are your league opponents?
My alt teams get rly hard leagues aswell... some 6stars teams i got are in 8th-13th place for some reason.. :D