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

Because whose blocks can be hyphen or underscore delimited, for example:

.site-search {} /* Block */
.site-search__field {} /* Element */
.site-search--full {} /* Modifier */

or

.site_search {} /* Block */
.site_search__field {} /* Element */
.site_search--full {} /* Modifier */