I read this somewhere a while ago but Facebook increments a counter for outbound traffic metrics before the browser loads the referred site, so if the user is quick to hit back/stop in their browser, the counts between Facebook’s outbound and your site’s inbound will diverge.
The conclusion in that particular article was that users were misclicking, and stopping before the traffic was actually received.
I cannot for the life of me remember which article it was so take it with a grain of salt, but I would personally put more weight in metrics that are counting inbound traffic.