funny, i was just reading an article by a somewhat pompous bicycle component engineer describing "the inadequacies of the quill stem and the consequent invention and proliferation of the threadless headset and bolt-on stem". according to him there was a precession action caused by the angled split plug design of the quill stem.

it seems to me though that the precession around the split bolt anchoring point is a consequence of stem manufacturers "cheating" on the outside diameter of the stem so as to accommodate all steerer tubes at the same time that steerer tube manufacturers were "cheating" on the inside diameter for similar reasons. although, to be fair, i imagine most of the cheating was being done by the stem makers inasmuch as the tube manufacturers have compelling reasons for adhering to standards.

anyway, as has been mentioned, it may help explain the looseness...