A number of factors can cause additional data usage on your ADSL line, below are a few examples

Network Chatter Up to 10mb per day of random packets caused by portscans, DNS, router update checks, remote pings etc. This will occur even if your PCs are switched off.

Skype  Your Skype client generates traffic even if not in use. This is because Skype can turn any PC into a 3rd party router for assisting in its P2P telephony functioning.

Stolen Username There are known hackers actively scanning insecure routers to obtain usernames and passwords. These compromised accounts can be used to steal bandwidth.

Wireless Hijacking If your router is wireless enabled, hackers in the nearby vicinity may be using your bandwidth for their own purposes, even if WEP or WPA is enabled.

Spyware, Malware, Viruses  Any of the PCs connected may be infected with the above causing your machine to become a zombie for sending spam or an instrument in sending denial of service attacks.

Software updates  Anti virus programs, Operating system updates are usually configured to update automatically when connected to the Internet.

These are just some examples of additional network usage that you may not be aware of. Check your computers software update settings, disable Skype when not using the service, make sure your network is secure and that your computer is free from spyware and the likes.