If 100% is defined as the most they can give under normal circumstances.
I think the "under normal circumstances" qualifier is bogus. To me, 100% is all there is. If you're being chased by a bear, that's where you give 100%. Under other circumstances, you may be running really fast, but it's not 100%. Just my curmudgeonly take on it!
I am with Kai on this. 100% is often a standard rating on engines or electric motors that can be exceded for short periods without damage.
Not beer related, but WWII fighter planes had maximum rating for the engines. There were also max combat ratings, and the top power rating was emergency power ratings for 5 minutes of operation. Emergency power broke a tell tale on the throttle, made of piano wire. The mechanics know there was some serious work to do, or that it was even time for a new engine.
Kind of like you might outrun the bear, but something is blown out the next day...