Final Fantasy Tactics Lesson

In the beginning of the game, they ask you to not only name the protagonist but they also want the protagonist's birthday as well. My character (who has been named Deya instead of just plain Ramza) has been given the birthdate of April 15th, and he started out with a lot of Priest abilites, and several other abilities as well. When I created other characters, they didn't start out with a lot. What effect does our birthdate have on the game? And does it have anything to do with the names of the random characters we're given at the start?
One of my charcters makes a horrible priest but can kick serious ass as an Oracle. Some characters weren't meant to be some things, just others. Does their Zodiac sign have anything to do with the job class they were meant to have? I mean, my Priest's percentages were way too low and nothing ever worked, but he makes it as an Oracle.
I guess I'm just asking to see if our birthday has any effect on our Ramza Beoulves.:) A long time ago I had a Ramza that was definitely born to be a Monk-he could take away about 300 hp with one single hit.
And why are certain characters unable to be some things but they can do perfectly well with other jobs?

Your main character's initial abilities and stats have nothing to do with his birthdate. At least, there's been no strong evidence to support this. What the birthdate does determine is his zodiac sign, which makes him either better or worse at dealing with units (ally or enemy) of other particular signs.