It might be relevant that the at seat staff Voyagers & HSTs are CC directly employed staff but (and this was the case recently but it may have changed) those manning the trollies on 170s were staff working for a contractor.
It's certainly the case that staff travelling on privs. etc. whilst allowed a hot drink on 22x/HST are not allowed one on 170s, supporting the idea that the 170 catering is more 'arms-length'.
Double Voyagers are pretty rare on CC and I accept your experience that both are not manned but on Virgin all sets were usually manned; I even got served in coach Q on a triple Voyager on a Chiltern Line shuttle triple Voyager on a Sunday.