That's a good first question. I came to the conclusion that the other managers where just NOT there. I never lose when I play a far weaker side and the other manager is present. Troll result occur against crap, half-abandoned teams. Just when the game decides to trick you. On the other hand, I regularly win against better sides in cup but the question remain about how well prepared was my opponent. In league, usually the best team wins provided the manager behaves seriously.
To answer your second question, my best win is a 3-0 second leg victory in cup (1-4 in first leg) against a team 1* better that me in average. In this case, the manager was there (on top of that, he was also a treble holder). And he probably ate his hat this day.