Home Firewall / Router data usage records less data than Xfinity... (Unifi, Sophos, PFSense, DDWRT)

I have a Unifi Router / Firewall. All network internet traffic goes through that firewall. It also records all internet usage per device, down to the hour. My firewall consistently reports 1/3 to 1/2 less data usage than Xfinity reports on their online data usage meter. I've seen other posts on this Xfinity Forum where customers report the same issue with SOPHOS, and PFSense firewalls. Why is the Xfinity data monitor reporting more data usage than our on premises firewalls? 

We can't all be wrong.