There are various factors that attribute to growth of the drivers.
Age is quite an important one. For example, Alonso, Hamilton, Hulkenberg and any other grandfathers of the track aren't going to see any progression as they're older than time itself and will just decline.
Callie obviously doesn't fit into that box, so i'd assume that the higher the level, the longer the level progression will take. Otherwise Max will (no pun intended) max out quite quickly, same with Norris. Tim at 62 has a lower threshold required to achieve the next level as there's more scope for his numbers to go up.
The Overall is based off a weighted average with pace being the main contributor. Tim's pace has probably increased quite well due to completing races and his slower pace level at the start. Callie won't have seen as much growth in that as it was already higher. I'd have expected some growth in at least one of her stats though, most obviously experience as that's not a subjective growth. It could be that she's peaked and this is as good as she'll ever get. Drivers have limits.
I guess taking control of a young driver yourself who you'd expect growth on and seeing if their stats increase or not would test whether it is tied to just drivers like Callie, or all drivers the player controls.
