Best I can determine is that they're having a cold or hot streak, though I haven't noticed a huge difference when using them in each state. My best guess is that this was included to make you spread driving duties around when you hire an additional driver. Hasn't really affected my choice though. Didn't like my first one, but I'm holding on to him for the multiple driver races. My second is twice the level of the first. I spam him into races all the time for some background cash farming.