I am the customer in question!!!
I was metered for 14GB on a day my house did not even have electricity, nor did their headend serving me. Also, I was not home, and it was not aggregate from the prior week, as I was on vacation the prior week. We had the derecho come through here and knock everything out in WV, setting us back a few years to the stone age. Yet on Saturday, while I was returning from Myrtle Beach, I miraculously used 14 GB of data, and the power was off most of the day at my house, evidenced by my ddns url being unreachable, phone ringing busy (I have SL telephone) and logmein showing my computer offline. My line monitor group also showed when the connection came back up that evening (we were lucky as we were only without power for a day).
I now use PRTG (on an ancient laptop) to monitor netflow from my Cisco router, and I have to admit, so far other than the powerless day, SL's meter jives pretty much with what PRTG tells me, maybe even a little less than what PRTG says.