You're not providing enough information. Router? Wireless? PC's? Etc.
Are you using this Netlimiter app on both systems?You should look into putting Tomato custom firmware on your router, so you can clearly see what everything is using, including possible freeloaders in your neighbourhood.Using tomato, you can get monthly bandwidth usage charts like this - »www.polarcloud.com/img/ssbwm100.png
DD-WRT does usage charts and real-time graphs as well, should someone prefer to go that way.