Hook Variations-Does Size Matter?

I posted this question on Crochetville, but if anyone out there has any input, I'd be interested to hear it. Why are hooks that are supposedly the same size, actually different?

I looked at 4 size G hooks. From top to bottom:

One bamboo hook by Clover, 4.0 mm
One aluminum hook by Tulip, 4.0 mm
One aluminum hook by Clover, 4.0mm
One (I think steel) hook by Boye, 4.25 mm

The actual hook parts are clearly not all the same size. They're also different shapes. And most importantly, they produce different size stitching. So I can't simply switch from one to the other mid-project and expect the same result.

So I got to wondering, what does the size actually measure? From what part to what part of the hook? And how is the angle of the hook from the neck determined? And what about the angle of the point of the hook? And exactly how does all this effect stitching?

Thus far, the answers remain a mystery, at least to me.