I think that's probably to do with the nature of who drives on which national speed limit single-carriageway roads. The majority of these roads are the kind of country roads that it would be insane to drive at 60mph on, so I'm not surprised that the proportion of car drivers found exceeding these limits is only 10%. Which single-carriageway roads do buses tend to use, though? I've no idea, but I wouldn't be surprised if it's a very different balance in terms of long, straight, wide ones versus narrow, twisty-turny ones. This means that the proportions are not comparable.