threshold not working properly

I compare 2 image which is the different only 1px X 1px, but when I tried to set thresthold 1 and 100 there is no different, the lib still can catch the different between 2 image. so what is the function of threshold, I expect if i set 100 which mean will more tolerance than 1
first image there is 1px X 1px red dot in the bottom of page

second image

but when i tried to used this setAllowingPercentOfDifferentPixels() -> there is a different result when I set 0 -> it will catch the different of image, but when I set 100 it will not catch the different