If your computer is the only thing using the internet then the numbers should be closer unless your ISP is counting something more as usage.
The program watches each packet that goes in and out of the computer it is running on. So unless you have something else on your home network, or someone else is using your connection (Like a non password protected wireless connection)
So there are 3 reason why the numbers are off.
1. Your ISP is reporting wrong, or counting something else as usage.
2. Something else on your network is using the net as well.
3. For some reason the monitor isn't seeing the packets going in or out.
My program uses winpcap to see the packets. If the program is running the whole time the computer is then the numbers should be right on track.

Shane