Questions for the Guru's

I understand it takes the right air intake to fuel mixture to gain hp and you also have to be able to exhaust same. Why is it you can take a carb that should have a 62 jet and motor runs rich, install a 60 jet and the rpms stay the same but plugs are correct color?

This is different carbs ,same motor.Now you take a carb that has a 70 jet and allows more air flow than above carb and decrease it to a 62 or 64 jet and still turn the same rpms. The 62 runs lean but the 64 are the right color.

Any help in this matter would be greatly appricated.