If you're in the Swadian faction for example, it goes like this. If you conquer a fief it's considered as "The Swadians have conquered xxxxxx", not "*player* has conquered xxxxx". Maybe if you're already pretty famous over the land they will think "The Swadians have conquered xxxxxx under the leadership of *player*". You see, you are fighting for your faction, claiming land in it's name, doing everything for it. You can read it out in your oath if any of you reads it. The king just needs a suitable Governor for the new piece of land and picks whoever he thinks deserves it. In his point of view his subordinates conquered the land but he needs a leader to govern it and that has nothing to do with who participated in the attack.