currval() within one statement

Helo

is it expected that the currval() changes its value between calls within 
one statement ?

Look the following call:

INSERT INTO ttt (a,b) SELECT currval('ttt_id_seq'), 'const' FROM ttt2;

Where the trigger before insert on ttt is defined and this trigger calls  nextval('ttt_id_seq').

I was surprised having different values of currval() in ttt.a

Is this the normal behavior ? Where is it described ?