Cajun,
We can run circuit-breakers with a combined rating which is greater than the incoming feed to the panel. It is based on a principle known as "diversity," which as you have touched upon above is the idea that everything will not be running simultaneously.
To take a simple example, suppose you have six wall receptacles (normal 15A types) wired on a branch circuit. The protective device will be 15 or 20A, not 90A. To wire the whole house on the basis that every outlet is loaded to maximum would result in a very expensive installation which would almost never be fully loaded in normal use.
That's why the NEC (and the equivalent codes we have in other countries) specify certain circuit configurations to take into account this diversity.
As Bill has pointed out, however, this is getting into quite a complex area which is best left to those with the necessary knowledge and experience.
