I think this may be because when you "conjure" summon an animal or elemental companion, you are moving it from its natural home to "you" to be your servant for a short duration. When you "evoke" summon the dagger, you are creating something from nothing and imbuing it with a personality, but again only for a short while. Both are "summons" but the way the summon is executed changes which school it belongs to. Also, it may have been to provide someone who is restricted from taking conjuration a chance to have a summon of some kind. In any event, that's my story and I'm sticking to it.