Why does BEM often use two underscores instead of one for modifiers?

Question

In BEM, I understand that with modifiers, two dashes makes sense so that you can distinguish the modifier in my-block-my-modifier with my-block--my-modifier

Answer 1

Double Underscore is used to define sub element of a block.

i.e:

    Text

Where main-nav is a block & main-nav__item is a sub element.

This is done because some people might name their block like this main_nav which will create confusion with single underscore like this : main_nav_item

Therefore double underscore will clarify stuff like this: main_nav__item.