I think I have an explanation for this. It looks like the driver you bought is probably a T20 driver from before the levels 21-30 was brought in.
I'm not actually sure if it is still the case, but it used to be the that a drivers level would not recalculate when experience was gained. So most fully trained drivers would show as level 18.
As soon as you added a point into driving abilities, the level will have recalculated to included the experience value of 20, pushing him up to a level 20 driver immediately.
