Oh Dixie, Dixie - good luck with that. Every single time I have pointed out there numbers did not match, I got the simpleton level 1 response: "Have you changed your password, etc.". Next they will forward your info to the next level where the answer will be "Our numbers are correct, it's not our problem."
Comcast certainly has a point - the amount of bot nets out there is enormous, but they are really trying to protect their legacy video delivery system. 300GB is an absurdly low level, so they are gouging as well. So far, I've had overages of 194, 6, and 187GB. I'm not stressing over it, I send the bill to my teens.
For me, I happen to have caught two of my teens using silly amounts of data (and it tracked with what comcast was reporting). I still don't trust comcast, but for now I'll tolerate it. The router I use allows me to radically throttle back their bandwidth 
Note to comcast: you're gone in 16 months. I can't wait to see what deal you offer me when I call to cancel.