How does mob tagging work?

i know a limited number of players can get exp/loot for mobs. i know cross faction damage can turn mobs grey. but if i tag a mob and then a same faction person does and then it turns grey why is that happening? for context im tagging mobs being killed by power farming bots. sometimes hp bar stays red sometimes it doesnt. when their group gets emptier i get them all. but if i tag a mob before they do should i not be in the exp/loot pool for that? am i being forced out of it somehow?