And u said you live in rural areas so that it would unlikely that some outsider is using your wlan..
But do you have neighbours living anywhere nearby? Someone could really accidentally use your connection, or just because they wouldnt know better that it shouldnt be done :-)
If you do not need WLAN yourself, disable it from the router or set/change the password for it if you need to have it enabled.
My cellphone uses my WLAN when Im home, do you have anyone there having a cell or anything which could also use the WLAN?
edit: ok, read a bit better. So your router does not show the traffic been happening at all what your ISP says there has been happening.. ummh, I dont know how/why your router could be wrong or what the heck would be the case here.
I certainly hope your ISP could and would actually want to investigate this further.
Hmmmm... Should you maybe try to like shutdown your router for 1 full day atleast, and then ask your ISP if they have measured some traffic for you for that day =)