How long has the team in question employed said drivers?
Level 6 is the point where drivers level up more quickly than managers, but managers are allowed to hire drivers of a higher level if the driver has been employed by the team in the past. For instance, if you level up to Level 8 but your driver is Level 12, and then you let him go and hire someone else for a bit, you can re-hire the original driver I believe as long as no one else has hired him in between, even if you are still Level 8.