Bandwidth usage tool inaccuracies 2/11/17 and 2/12/17?

With Cox deciding to bill for data usage over 1TB (my account is temporarily at 2TB) of transit, I have been keeping more of an eye on what Cox claims I've used. I connect my cable modem through a managed switch before it gets handed off to my edge router. I collect the switchport stats in an industry standard fashion that has been proven to be accurate for years.

The cox system is regularly off, but generally by a small amount and usually in the favor of the customer. This would indicate the system calculating usage often doesn't receive data, is down, or has random issues.

For the beginning of this billing period, however, it's pretty ridiculous. Since my job is working with 'big data' I do often have to push a lot of data around. I used quite a bit of data on 2/11 and 2/12, but cox is showing quite a bit more.

On 2/11 Cox shows 281.65GB, actual usage was 130.96GB (in/outbound combined).
On 2/12 Cox shows 211.01GB, actual usage was 117.42GB (in/outbound combined).

Seeing as Cox's goal is to charge $10/50GB over 1024GB, an almost 250GB error is disconcerting. 

For those following along at home, yep, Cox's error for two days is 1/4 of the amount of bandwidth they allow users to use in a month. 

Anyone else noticing large or unusual usage on those dates?