Other than the obvious difference in stroke, skill, and knowledge of the game, I would tend to go a little deeper. I see lot's of amateurs that play so good at times, it becomes tempting to ask "Why doesn't this guy go pro?" or in other ways tempt one to think that "hey, this guy could beat Mika the way he's playing tonight!"
So in that respect, in my opinion pros and amateurs all have a good game and a bad game. But the gap between a pros good and bad game is not as wide as the amateur. A pro that is a danger to run a 6 pack on any given day might have a really bad day and have a hard time putting together anything more than a 6 ball run. Whereas an amateur that can also run a six pack if things are good, but at his absolute worst might have a hard time putting together anything more than 4 balls.
Take those differences over a tournament that lasts 3 days with numerous matches against world class players, and the highly likely chance that during the course of the tournament any given player is going to have a 'bad' day, it all comes down to which player is still a threat to pull off a win even while having a bad day.
dave