I have no idea how the game picks which job when you first select a mob/monster/instance. I have noticed that archer, brawler, wizard are all level 8, and now my ninja is what I am changed into when ever I click on a fighting NPC and he is only level 5.
The only time it picks one of my higher level jobs is when I go into a higher difficulty instance (Level 5 or higher) or when I am doing a specific job quest example a brawler quest auto-picks brawler.
I have no idea how they have the game set up for auto-picking.
Personally I dislike being automatically changed to a fighting job when I click on a mob, I often click to see what the difficulty it preferring to be in my adventurer mode when exploring and running around.