the added rest days are allowing for more growth. If one uses proper form and rep speed like DC recomends (or any HIT advocat for that matter) then 1 set is all u need. DC's split is a higher frequency HIT routine.
Muscles should be trained Intensely, briefly, and infrequently. by you training arms infrequently (even less the DC recomends), and with High intensity low volume, you are minimizing the inroad into recovery ability but maximizing gains by giving them time to recover AND THEN grow. One thing that people dont relize is that training a muscle when its recovered its not always a good idea. you are only allowing them to compensate from last workout, but not over compensate (or altleast to its full potential). You are taking advantage of this scientific fact without even knowing it.
and also as Excel said. they are also being stimulated from other compound movements.