There could be several reasons. Here are the most common ones:
Your ISP counts all bytes sent or received to you. However, some packets are completely lost in transit or delivered with errors. These packets cannot be counted by DU Meter. Moreover, if you have a firewall or router device, that device can and will drop some packets, so they are never delivered to your computer, therefore DU Meter won’t count them.
Your computer generates some broadcast traffic, which never leaves your local network. Even if your computer is connected only to your ISP via DSL or cable modem, broadcast traffic will still be generated, but it will never arrive to your ISP. Moreover, if you have other computers in your local network, communication with these computers will be accounted for by DU Meter but not by your upstream network provider.