Maybe the reason why some people get accurate readings some days and overestimated the other day is because the modem's MAC address have been cloned for that day. The people that use hacked modems to connect to Cogeco's network can change mac everyday to avoid getting caught.
We monitor that (seriously, just because you haven't gotten a knock at your door yet doesn't mean we don't know where you are) and have a process to remove them from the equation. -- you can lead a horse to the water but you cannot make him drink...you can put a man through school but you cannot make him think --ben harper