A number of points here
1) the wifi monitor on the PC is only going to monitor just that traffic between the specific PC and the router
2) if you have a BT Home Hub, then that actively looks for firmware updates from the BT servers and automatically updates itself. It may do this a couple of times a month. Thats extra data
3) again, if you have a home hub, third parties may be loggging into your open BTWiFi or BTFon channels. Thats more extra data
4) there will always be a degree of redundancy in the data sent over the internet due to error checking and correction. Depending on the router, phone line and max packet size this can be quite significant