About 3 days ago I too noticed that I was losing/using much more of my 10Gb than was right. I also started to have problems such that I couldn't get to websites that I'd been getting to for years previously, and my speed was slowing down considerably.
One solution was that I did a hard reset of my router (a Linksys E-2500). That made it so I could go to all those websites again, but it didn't stop the usage meter from being wrong and saying I was using more than I actually was.
So, last night I installed the dd-wrt firmware for my router and lo and behold - the usage meter isn't adding up incorrect amounts of usage anymore! I was getting a bit PO'd that it was adding up usage during the free-time...so to test it, last night I did a good deal of downloading (probably ~2Gb of stuff) and the usage meter didn't add on a few hundred MB as it was doing before I 'upgraded' the firmware in my router. My speed is back up to par and the usage meter didn't budge!
YMMV, but I would try the dd-wrt route if nothing else works.
Microsoft follows standards like caribou follow migrating salmon.