@majoritasdev I asked the live help, apparently this can happen if it’s a follower campaign. I don’t have the exact explanation handy, but in those cases, the metrics can come from the promoted tweet OR from the “Who to follow” section of twitter. In this case, the metrics wouldn’t be associated with any specific promoted tweet.
I asked if this was the only situation in which the metrics from followers wouldn’t add up to the campaign metrics, and the support person said that yes, this is the only way this can happen.