I've never done a triathlon (just a duathlon) so I can't say I'm much of an expert. I've just picked up on some of the lingo since many of my cycling buddies do tri's. So, with that disclaimer, here's my understanding of types of triathlons.

Triathlons are generally categorized by distance with the main types being Sprint, Olympic, Half-Ironman and Ironman. Since the sprint is shorter, it will favor speed over endurance. So, you might get some fast people who specialize in those because they can do better. But I think you also get a lot of beginners that do sprints because they want to start short and build up to the longer races. Because of this, the main pack might not be as fast as you would think.