Think of it like real life. You have no specific level. Sure you're so many years old but that doesn't make you a level 50 human. Instead what are you good at? If you level Ninja to level 50 but never did a day as Black Mage, you're still going to be a level 50 Ninja and only a level 1 Black Mage. So the level of a job is separate from the rest of the jobs and unique to that character.