add_filter confusing

I am confused about filters, why does this work:

function get_the_author_description2() {
	return 'test';
}
add_filter('get_the_author_description', 'get_the_author_description2');

but this one does not:

function get_the_author_firstname2() {
	return 'test';
}
add_filter('get_the_author_firstname', 'get_the_author_firstname2');

Confusing!

I know these are depreciated functions, but the plugin I am trying to override is using them.

Thanks