They might have left the game? or they could have been killed again in the time it took you to run to where you killed them to get the tag, other than that who knows?
probably because one of them denied a tag before you killed him... it happens all the time to me. everytime i kill 2 guys near each other, the second guy i kill always denies the first guys tag. it sucks, but there isn't much you can do.
the moment you killed him, his butt buddy most likely got the deny immedieately, leaving only his dog tag. I know ive gotten an immediate deny running next to a teamate multiple times