COX data measuring tool is off

I have been going back and forth with COX for about a month now trying to explain that their traffic analyzer is off but have only been told that it can't be because they work with a "third party" who's own 2018 report show that they DO have a chance over over counting traffic used. There has been no report for 2019 released to the public yet so it is impossible to verify if they fixed the issue. The problem is that I've spoken to my router manufacturer who has assured me that as long as my router firmware is up to date the wifi traffic should be accurate to the KB. After my own research I've found that the only way to be sure of the traffic used on wifi is to take the measurement from the router (which I do) and nothing else (COX uses the modem) so I'm starting to think that they are sending a fair amount of data to my modem (my personal modem not theirs) and then charging me for it, which they shouldn't do per their support reps, or their measuring tool is off which they wont even consider. I'm curious if anyone else has run into this issue or even just COX's refusal to engage with their paying customers about possible issues.