Actually, only half out of all difficulties (both vol.) give gallantry equal to half the amount of hallmarks..
BGWiki page says max gallantry is:
V2: 5/25/50/100/150
V1: 100/200/300/400/500
As for reasons? Patterns are pretty. :3 Let's go with that. Also, VD now gives double the amount of hallmarks that it did at launch... Forget exactly when they did that adjustment, or whether gallantry was affected at all tho...