Class vs job?

[Xia_Thas]

How come jobs like samurai don't start out as, say, a swordsman class... or something?

The easiest explanation for this is that all of the Expansion jobs don't start at level 1. MCH DRK and AST started at 30 which is when you'd do the Job change quests on the base jobs. So because of that you just started with that Job's soul crystal. Same with RDM, SAM, GNB, and DNC starting at 50 / 60.

The only added Job that had a base job was Rouge -> NIN since it was added in ARR before the expansions.