In addition to the points addressed by other posters, the difficulty of winning the cup is relative upon what level a manager is or more specifically what the financial awards for each match is at a managers level.

Financial match award, relative to win bonus is lower at higher levels so several manager at higher levels can't afford to max that win bonus as often as they could at lower levels. So if a manager can afford to put win bonuses on max; they'll have an easier time advancing into the later rounds of the cup, even if the opposition manager has higher quality.

As already mentioned by others, token buyers often leave their quality to high so get placed in higher level cups. But having power-trained players and the win bonus on max can often overcome higher quality teams at higher levels if the level gap isn't to large. Where the difficult arises is late in the tournament when managers start believing they can "WIN" the cup, so a free player that might not max the win bonus in the first few rounds might be doing so in the last 2+ rounds.