I was taught that the reason a higher ampacity is permitted for these service and feeder conductors is the intermittent nature of dwelling loads and their inherent diversity. The key to sizing the feeders is that they must carry the entire load of the dwelling unit in question in order to make use of the reduced sizing. There is a much lower likelihood of the load on a garage feeder being diverse enough to make any reduction in feeder size a safe practice.
--
Tom
