Had the exact same thing. Let me tell you my story without overcomplicating it.
Back in august, Counter strike had one of it's biggest tournaments of the year. That being said, it was in germany, therefore streaming was late @ night, from 4amish - 2pm in the afternoon.
Anyways, if u link your twitch account and watch the stream, you can get gun skins in game, etc, etc bla bla bla.
The tournament ran from Thursday > Sunday.
here we go.
Go to bed, run the stream on mobile connection. Wake up in morning, either 1) turn it to high and watch until over, or 2, goto work and keep it on mobile connection.
Long story short, from thursday - sunday i used about 20gb per day. For the rest of the month, not streaming, or doing ANYTHING.... the usage portal continued to pull in 18gb-22gb PERDAY. Without even remotely doing anything as intensive as the streaming listed above.
Of course, tek savvy gave the usual b.s (check if someone is stealing your wifi, bla bla bla)
I was out an entire day and somehow still produced 20gb of usage that day.
Anyways, 301GB later, on august 26th, i had to literally TURN OFF my modem for the rest of the month to avoid overages.
As of september, i am now on zap the cap as I just don't trust the portal anymore. It's happened to me as well a few months prior, although it's quite hard to produce evidence as to how the portal is lying or wrong.
Sorry for the messy post, although i am a firm believer that the portal usage tracker is completely incorrect and it's a shame.
---edit---
just to put this into perspective, i looked up the usage.
The tournament ran from August 14th till the 17th.
8/14/2014 18.3
8/15/2014 21.41
8/16/2014 17.84
8/17/2014 18.96
And here is the usage thereafter. I would love Tek to explain to me how the usage below was similar to the usage above when I barely streamed at all and was out for one of those days.
8/18/2014 18.68
8/19/2014 19.82
8/20/2014 19.45
8/21/2014 19.73
8/22/2014 18.51
8/23/2014 18.67
8/24/2014 18.05
This is where i had to shut off my modem for the rest of the month.
8/25/2014 0.92
8/31/2014 0.01