The reason you are in a high level Cup is your Team is THAT MUCH above the average for your level. I know it doesn't seem fair, but if you have a slightly above average Team... you're in your own level Cup. (When I say Team that means best 14)

So, if you power train or always have 5* or better best 14... you will always get a cruddy Cup. I know I sure have! But my 35T teams have won the Cup more than BvB *because* they are only a bit above average for their level.