DavidR is on the right track. There was a post quiet awile ago about the same issue. The counting software was reading the data going through web shield and, darn I can’t remember what else was being counted. But most ISP have a feature on their web site, within your account, to see what your usage is. You can try to see what they have recorded for your usage and compare.

Maybe someone else can remeber the post and how they got around it.