I think our theories have some flaws in them, or the game has some code we aren't taking into consideration.

I had a Day Trader level 1 at $105/day marry a Boardgame Designer level 2 at $105 a day. They were doing well, getting the same pay, and I made them have a baby. Dad got a promotion while mom was carrying the kid. As soon as she put the kid down, she went back to her job Day Trading. Her next promotion, to level 2? Now her salary is only $90. Either a bug, or we don't know enough about the jobs to see everything. To me, it looked like she got demoted for not working for two years.