Ну вот стандарт С99 кое-что весьма разумное пишет, к чему именно прикапываетесь:
If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined.