How many times had you tried to pitch before that day? My guess is less than 50 times. It takes several dozen practice runs before, really, the ideas gel in your mind, the questions people ask are all predictable, the confusion points are already planned for and navigated around. This is just an example of how everyone feels when they've done 'the pitch' an order of magnitude too few times.

I think it's also worth pointing out, listen to just the very next person who comes up to speak. PG hands a big gift-wrapped present to the speaker, "with that in mind, please, tell me precisely what you do" and look what the entrepreneur does... launches into a thing about something Bill Gates said, and you can see PG just going right back into that pain zone.

So I think it's a terribly common problem, and really, it's something that pretty much everyone works through manually just by getting out and giving the pitch over, and over, to different people, gauging their reactions, their body language, their questions, and just adapting, refining, sharpening that pitch to the point where it really crystallizes exactly the best way to communicate how what you're doing really makes a difference.