Whenever you watch cup competitions you always see loads of teams & players who look good enough to compete in the Premiership.
Fuck, there are teams & players who could compete, but only on their best days.
The difference between players at the highest level is how often they can play at THEIR highest level. The very best do it 8 or 9 games out of 10. When the fa cup rolls round they up their game & give it everything they have.