Probably as easily as an ISP can advertise an unlimited service but apply limitations to it which it won't disclose to those being limited.
I know of at least one perfectly legitimate way to max out any speed line you can get 24x7. Perfectly legal yet VM would soon boot anybody off who persisted with it. As you can see from the daily stats some (non-UK) ISPs deliver what they advertise...
http://www.majestic12.co.uk/