I'll quote a Mod to answer that:
'... something that only occurs with drivers that have existed before August 2016 and were previously signed to a team before that date, when they get trained and gain +1 in any attribute, their sudden level hop (usually 3 levels) reflects their new level in regards to the new format of iGP (there used to be an old format that was not mobile friendly).'
This and the driver was from the old version of the game and unemployed until hired by this team. The levels were calculated differently back then and only with the first training success in the new version the level gets updated according to the new skill set of the current version of the game.
I remember when i played this game on my old account, i was lvl 7 and i had a lvl 9 driver in my team because i trained him. Maybe that was the case or something else?
