First, here's a look at the Chrome Extension (if you want it, go get it here!):
The above example is for crowdtangle.com, and it shows all the folks who have shared links to our site on the public social platforms we track. Everything under "Top Referrals" comes from CrowdTangle data and is the same information you would find in any of our other products.
The top bar, however, comes from a different place within Facebook. It is meant to represent how many interactions a URL has gotten across all of Facebook, not just public posts, even including shares via Messenger.
Sharp-eyed readers will notice that in this example, the totals row features fewer interactions than the sum of the links listed below! How can that be?
The answer is that the data comes from different sources, and we have different ways of calculating things. We at CrowdTangle don't have any control over the numbers that drive the totals row, but we do know that any changes to a website or the canonical URL (e.g. moving from http to https, or changing the address after it has gone live) can make it seem like the site is an entirely new webpage that starts aggregating anew.
So if the top row of numbers isn't larger than the sum of the bottom links, our best guess is that it came from a change to the site's URL that made the Facebook system think it was an entirely different site. But you can be certain that the bottom totals are accurate based on the posts we're watching within CrowdTangle.