All of this long post can be sum up in 1 sentence: Because the model hitbox was not correct to where it should be.
The setting change the hitbox based on the model. If set to 0, the hitbox is the regular CS 1.6 player model hitbox.
This is the problem with the model, not about the setting.
For example if the model index was turned on:
- CSO Heavy Zombie got head hitbox at his leg
- CSO Light Zombie got hitbox even lower and you can't even shot her head when you are in duct because there is no hitbox there.
- Classic ZP Model don't even have any hitbox other than body.
I personally think it is a way to counter those using aimbot since they will aim to the head hitbox of regular model and it will miss 9/10 times and player using cl_minmodel 1 to hit easier can't hit because different hitbox