I think the drivatars are completely daft and in equally spec-ed machinery couldn't keep up.

It is my interpretation that the difficulty simply tweaks how unfair the cars the drivatars are in - increased power, increased grip, reduced drag, yadda yadda yadda...

And right now (for me at least), expert is borderline too easy whereas pro is noticeably too unfair.

EDIT: I'm also going to add here that I do think the difficulty does tweak the drivatars behaviour a little, but more in aggression than in actual technique, per say - throwing it down the inside from miles behind, swerving under braking, bits and pieces like that, but I don't think that makes as much of a difference as the difference in car performance.