Data Usage General Question/Concern

Up until last month, my data usage on Cox 100/10 plan sat around 650-700GB usage.  This is on one PC that gets used throughout the day - and very marginal streaming through Netflix.  No consistent downloading occurs on this PC, no WiFi devices are used, nothing has changed and it is virus free (before we go that route).  

My concern is - the last two months my data usage has skyrocketed.  Last month I sat at around 1050GB, so I paid the price for the overage.  This month, nothing has changed in terms of usage, but I found that I somehow went to 1400GB used with 4 days to go on the billing cycle.  I am versed in technology, having worked for Cox in the past, I am versed in their internet as well and the capabilities it has.  

With all that being said, I tested the last 30 days of usage up until this point on both my router/modem and through the ethernet data tracker.  With 6 extra days added from last month it they all show around 660GB of usage, even with the extra days calculated.  How in the hell is there a near 800GB discrepancy between the 3 different trackers.  The only one that is different is the one that is going to charge me overage charges?

Has anyone else or any technicians here noticed any problems with the data usage tracker?  I know they outsource for reliability and accuracy checks but ... something has to be going.  I'm feel like I am getting scammed with the only option being "pay us more than a $1 per Mbps because we see you use more data".

Thanks,

Jim.