Solved

Training progress

If it's your two active drivers you're talking about, you have a L13 team, one of your drivers is L13 the other L14. I guess the driver who has lower progress is the L14 one.

In a nutshell, the higher the level of the driver the lower the progress in training. When they become higher level than your team they only progress 3% each training session.

Edit: Drivers at a higher level than the team also usually display with red stars.