If you have smartphones, tablets or home theater receivers, you may be downloading a lot more than you think, or than you can monitor from your working PC. All of these devices can stream data from the Web, including in the background when they are not in active use. The only way to measure total usage is where your network connects to the provider's network, which is at your data modem. If your router doesn't keep track of data flow, you would need to insert something to do so between it and the data modem.
A VOIP phone from a cable provider may also count toward your data total (find out from your provider), but since it connects through a separate modem, will not be counted by your router, or any device inserted between it and the modem.