In what manner did you check your usage?
Depending on the "shape" of your Network there are potentially many usage paths.
As an example, my Router has a number of optional internal services that can be enabled. Those services, if enabled, can use considerable data and it would not show up at any of the normal "measuring points" because it IS at the router level .. just in front of the Modem.
Further, Router settings and security will enter into the 'usage path equation':
>Is WPS disabled? (has know vulnerabilities)<
>Are ALL Guest Accounts disabled?<
>Has the Routers GUI's access username and password been changed from the default and only known to a single designated user?<
>Have ALL wireless frequencies (2.4GHZ or 2.4/5.0 GHZ or 2.4/5.0 + secondary 5.0 depending on Router) ALL have had wireless encryption enabled to at least the level of WPA2-PSK [AES]?<
>Has the Router had its firmware manually updated the latest release?< (this is particularly important in certain Netgear, Linksys and Cisco brands that have has known vulnerabilities<
>What is the 'character" of your usage download/upload ratio?<
Some changes have been introduced in both software and drivers over the last few months.
AVG Antivirus now collects and 'shares' data with its publisher. (read increased upload)
Some Nvidia video card/chip drivers now also collect and share user data. (more upload)
Win10 (unless on a declared wireless connection will not only auto-update but will also collect and share "telemetry" with Microsoft. This will result in massive download and upload activity.
Currently there is a Update Roll-Up ( KB4015217) that not only is very large but for many users is starting/stopping/restarting. In addition to that there is now being rolled out the Creators Build version of Win10 and that will be a very large update as it is a complete OS rewrite.
There are numerous places for data to 'leak" and few users have a true and accurate picture of all of the potential connection paths.
It seems it is easier to just accuse their ISP of stealing data.