Twice I have recently updated comps from win8 to 8.1 and have seen a 8GB spike in my usage both times. 8.1 is 3.6GB and if you add 400MB for other updates you get approx 4GB.
I have also recently synced 13.8GB to my Google Drive and saw a 20GB spike in usage for that day.
Today I did a test of a 3.17GB upload to Google Drive and that has just given me a spike of over 7GB.
I am the only regular user and don't do downloads of music, films etc and no torrenting.
The spikes seen on my billing are pretty obviously related to known abnormal traffic usage but they just don't match the actual traffic.
This has caused me to up my plan so I hope it hasn't unnecessarily cost me.
Maybe a kind Orcon guy could take a look?