Data cap.

Okay so I had no clue there was a data cap. I know for a fact I download a ton of games, movies, streams, and what not. So on my router I like to monitor my data usage just to see it and for this month my router has recorded 855.69 GB. But on Comcast it says I've used 923 GB. So either my router is reporting the wrong number or Comcast is ripping me off with 100GB. Anyone else monitor their data and notice this? Or does Comcast also count in cable as data usage? Also when did we have data caps? I remember last summer I've downloaded 1.5 TB several times and never got a notification. My area is California, bay area.