Well, people can have their modems on and downloading during the day, but if you forget about that fact for an instant, remember, alot of people come home after a day's work, switch on their pc and internet connection at home and surf (or download)
That COULD explain it.... it also could be possible that there is noise... have you checked your signal strenght and noise at all during those times?
Then again, who knows...
Hell, my gran on a scooter with a memory stick is faster than Sentech's MyWireless!