Correct. But they are almost certainly calculating it correctly. There's the data, the headers, any background traffic. Gaming does this a lot, btw. Tons of tiny little wasteful packets and the system constantly asking for updates/pinging/etc. Downloading a torrent is even more crazy - you can send an astonishing amount of traffic back and forth by the time you get even a single small file. Add in some dropped packets when the telephone is being used and there you go.
When I had DSL, if I started talking on the land line, my speed would drop into the dirt. I could hear static on the phone as well. DSL works over amazing distances, but he might be struggling to get even 256K. Note - old wiring in the house also does this. Most proper installs run a new line from the pole directly to the computer. A lot, though, especially if you are in an apartment, do not. Given how crappy DSL generally is, I can see a 20% overhead quite easily.
But, it's certainly not ethical, either, to be counting this towards a cap, especially if there is no non-capped option available. (well, there always is - via their business service, but that's a whole other wad of butt-hurt).
Note - the OP really needs to get cable or something without the insanity. DSL is always the choice of last-resort.