the o2 mixture is evenly mixed throughout all of the exhaust gasses, you can dump out all you want and it will still get the same reading because its a ratio of air compared to burnt fuel. (whatever fuel gets dumped out, the same of air is going out with it, keeping the ratio the same.) kinda like a glass of sugar water mixed evenly, theres still the same amount of sugar compared to water no matter how much you dump out.