Its about classification and which comes first in the order of things. Seat is first and saddle is a subset of saddle. A saddle is a type of seat.

As far as the tube or post, its a matter of the general classification, and not the specific. Why its used? Probably because it does not need that much "precision and clarity". Its understanding is "understood".

Not bike related but if you want to learn more about a subject you take lessons, if you know less about anything you are a moron!

This would make a good tee shirt.