Why is a simple simmer, evidenced by 'fish-eye' bubbles, considered insufficient for disinfection?

Answer

It does not confirm that all the water has reached the critical temperature

A simple simmer or the appearance of small bubbles is explicitly insufficient because this visual cue does not confirm that the entire volume of water in the container has reached the required effective pasteurization temperature.

Why is a simple simmer, evidenced by 'fish-eye' bubbles, considered insufficient for disinfection?
temperatureSafetydurationwaterboil