why I can call pg_stat_get_backend_idset directly

Hello

Iam play with SRF function and found difference between behavior system
function pg_stat_get_backend_idset and own function in plpgsql;

aopk=> \df pg_stat_get_backend_idset
 Datový typ výsledku |   Schéma   |           Jméno           | Datový typ
parametru
---------------------+------------+---------------------------+----------------------
 setof integer       | pg_catalog | pg_stat_get_backend_idset |
aopk=> \df aa
 Datový typ výsledku | Schéma | Jméno | Datový typ parametru
---------------------+--------+-------+----------------------
 setof integer       | public | aa    |
(1 řádka)

I can select * from pg_stat_get_backend_idset();
I can select * from aa()

I understand, but I can call too

select pg_stat_get_backend_idset();
 pg_stat_get_backend_idset
---------------------------
                         1
                         2
(2 řádek)

This isn't right for srf function, or else. I wont write own srf function
with equal feature - similir like MSSQL procedures. It's possible?

Regards

Pavel