Why grantor is owner in this case?

Hi guys,

I have a question about the grantor. Why the grantor is owner in the following case ? I think it should be postgres(dba).

postgres=# create user u1;
CREATE ROLE
postgres=# create user u2;
CREATE ROLE
postgres=# set session authorization u1;
SET
postgres=> create table u1_tb1(a int);
CREATE TABLE
postgres=> reset session authorization;
RESET
postgres=# grant update(a) on u1_tb1 to u2;
GRANT
postgres=# select attacl from pg_attribute where attname='a';
attacl
-----------
{u2=w/u1}
(4 rows)

From attacl "u2=w/u1", we can see the grantor is u1, but in fact the grantor is postgres, the dba of database.
Does anyone know why ? Or is that a bug?

--Dongni