Cox Data Usage meter seems way off

We were told we'd gone over the 400 GB cap for the "Ultimate" package (leaving aside that "Ultimate" really shouldn't have a cap, and should be a big part of the perks for paying 4x what basic internet costs). Since I hadn't been tracking it, I couldn't say that we had not. I looked at the graph and it's very suspicious. It seems that we apparently *never* use less than 10 GB, whether or not we are actually home or doing anything.

Of course, this could be because someone else is accessing our wireless somehow without us knowing. So I have set up a meter on our router. The meter says we've used 16 G over the past 3 days (since this includes a weekend, this is in line with what I expect, we are fairly heavy users which is why we have the "Ultimate" package") But once again the Cox data meter shows more than 10 for each *day* with a total of 33 G used! It's not a simple double charge -- I've been watching it each day and it's just a coincidence that it's presently right around 2x.

All this happens the month after some Cox service personnel worked on the local neighborhoods network. Coincidence? Possibly. Possibly not. Who are we supposed to call about this? Regular technical support is great for things like setting up a local data meter on your router (thanks, by the way) but it doesn't seem like they can do anything about a broken data meter on their own.

In case it matters, we bought our own DOCSIS 3 modem that was supposed to be compatible with Cox. Is that a possible source of metering trouble?