Czuch Chuckers: Is there really only one best play for every given situation?

Yes, but. With perfect knowledge there is almost always going to be abest play, the only exceptions being when there's a tie. But we don't have perfect knowledge and nor do the computers .. yet.

Maybe what is a good play against another computer wont be a good play against an amateur?

Let's change it round a bit .. a good play against an amateur might not be a good play against a computer

Not only against an amateur but even against the different robots (though that level of subtlety in way beyond me). The "perfect" moves as reported by any of these computers is only perfect against itself. However that doesn't mean that they are wrong, it means that in certain situations they'll be slightly less than optimal. There's a huge amount of agreement between the top robots.

Against amateurs you'll find that there are definitely situations where you can make what would be a bad play against a computer or top player. For instance I'll leave a vulnerable blot in certain situations. The computer would jump on it with glee but it's safe when played against some opponents because they are too scared to do the hit. And that blot then gives me more options to hit them, or has them making poor choices in order to avoid the danger that it poses. Generally, with a player who puts too much emphasis on safety, you can get away with, and should try, things that would get you a slap on the wrist from the computer.

Putting it the way that you did, where the good play against the computer is bad against the amateur, is less frequent, I'd say. The good moves will always be good moves, even if not the best against that particular player in that game.