In my 4th season and in every season I have lost in the cuo to lower ranked teams. It's two legged so harder to say it was chance and for it to happen every season is quite unlikely.
Tactics don't explain the results. I lost the champions league final only 2 days ago despite dominating the game but it was a fair and possible result with two evenly matched teams.
In my experience cup games are just random troll results that have zero to do with the quality of your team. Just really annoying is all.