as far as I can tell, you set this variable as subcategory, hence why you can't un-default it

There must be a default value! If you want to change the default value, just tick another box and the other tick will go away.

Subcategories require a default because it otherwise doesn't know what to display when you click on the category it's assigned to. only regular variables can have no default, as a default for a regular variable automatically hides all other runs.