Russb, Do you remember what other items you have seen this application on ?
I can see a THINNER bolt breaking ... but your impression of a longer bolt holding tension better works with things like head bolts... but hanging the ' extra ' length outside the clamping area does not seem to comply with that type situation. And, if this were a thinner bolt it would introduce a better chance of vibrating... particularly hanging out unsupported that way...
I think there must be some other explanation....