The viewing angle depends on how close you are to an object. You're talking about viewing height which is not the same thing. Things at a distance look smaller than things close up. If you're standing 20 feet away and I take a picture of you, you'll look different than if you're standing 2 feet away from me. They also appear to flatten out at a distance, whereas up close they are more 3 dimensional.
The calculation was done based on the ratio between the rim-to-fender distance against the rim diameter.
If you thought I just literally took the number of pixels and converted into a measurement, that's not the case at all.
In terms of pixels, the gap on the green car is more than double that of the yellow car (94 pixels : 43 pixels = 2.19 times);
The adjusted measurement is merely 19% different (10.6cm : 8.9cm = 1.19).
Also, the greater the angle, the smaller the gap will appear. So the green car's tire gap would appear smaller to the eye if you are looking up at a bigger angle...