A proper setup in my view is one that helps you generate the best compromise in safety, comfort, overall power output for the legs and aerodynamics. In that order. Even among people who are the same height with identical arm, torso and leg lengths, one fit may not be desirable for both. For example, it is possible that a 25 year old racing wiz would have a drastically different setup from an identically sized man 30 years older. The 20 something guy will likely have overall good muscle tone, small belly, decent back, good cardio. The same may not be necessarily true for the other man.
There is also experience as a factor. Leaning over at a 45 degree angle is not comfortable for everyone, especially not when they are starting out. As a really obvious example, if you have a large pot belly or beer gut, this is a problem. I remember someone talking about bit fitting as a side story to the Tour De France on TV where they stated that beginners are better off with higher handlebars, wheras professionals/advanced amateurs will get more power/better aerodynamics with lower handlebars.
http://www.youtube.com/watch?v=FAl_5e7bIHk
With triathlon/time trial bikes, the issues become even trickier. I found a good article on this:
http://bethelcycle.com/articles/debunking-three-tri-bike-fitting-myths-pg287.htm
I also forgot to mention the scenario of someone who does both triathlons as well as conventional road biking. Because of how the typical setups for both sports place different emphasis as to which leg muscles get used more, they might also have different setups in their non-specialty bike/non specialty bike setup if they have more experience in one sport over the other.