From what I know stars are an indication of level of quality to manager level:
For example, a level 10 manager would see a level 10 DR, in this example, as 5 stars, but a level 11 manager would see them as 4 and a half. A level 20 manager would see it as no stars at all.
As your manager experience increases and you progress, the level of staff and drivers you can unlock becomes higher as well. That’s why stars decrease as you level up, because your staff aren’t leveling up with you. It’s a good idea to get new staff with every level up to level 10, when the progression rate becomes lower
