Equal test on strings doesn't ignore trailing spaces if the function 'lower' is applied

I have to apply rtrim explicitely if the string to
compare has been converted to lowercase :

dip=> create table testlower ( t char(50) );
CREATE
dip=> insert into testlower values ( 'Aa' );
INSERT 1257077 1
dip=> select t from testlower where t='Aa';
t
--------------------------------------------------
Aa
(1 row)
dip=> select t from testlower where lower(t)='aa';
t
-
(0 rows)
dip=> select t from testlower where rtrim(lower(t))='aa';
t
--------------------------------------------------
Aa
(1 row)