Why does fn_my_permissions(NULL, 'SERVER') return different results depending on which database is in use?

Question

fn_my_permissions "returns a list of the permissions effectively granted to the principal on a securable."

use master
select * from fn_my_permissions(NULL, 'SERVER') // 25 rows

use some_other_database
select * from fn_my_permissions(NULL, 'SERVER') // 25 rows

use a_third_database
select * from fn_my_permissions(NULL, 'SERVER') // 0 rows

Why would my SERVER level permissions depend on what database I was using?