Hi
couldn't see page 2, it came up with a message about invalid page codes, then went blank.
Anyway, it seems logical that you'd get lower voltage readings at a higher heater voltage because at the higher heater voltage, the sensor detects methane rather that CO.
How have you tested it in CO? As hero999 suggested, you could burn some charcoal and expose the sensor to the fumes (though I recommend you do this in a well ventilated area)
Ideally you need to compare readings from the sensor being exposed to CO, and reading from the sensor being in 'clean' air (ie sensor outside) so that you can see the difference between the two. This will give you an idea of when CO is present, but because you don't know the exact quantity, you will not know whether it is a 'safe level'
Oh, don't expect to get exactly the same results every time, because you cannot accurately control the levels of CO in the air, and they may vary slightly.