vacuum & free space map

Hi,
I'm not understanding why a vacuum full clears a FSM information while
a normal vacuum does not. On a table with fillfactor = 50 I've got:

testdb=> vacuum full respi.pull_status;
VACUUM
testdb=> SELECT * FROM pg_freespace( 'respi.pull_status', 0 );
 pg_freespace
--------------
            0
(1 row)

testdb=> vacuum respi.pull_status;
VACUUM
testdb=> SELECT * FROM pg_freespace( 'respi.pull_status', 0 );
 pg_freespace
--------------
         4096
(1 row)

So why is vacuum full not considering the fillfactor and clearing the
FSM? Or am I misunderstanding the results from pg_freespace?
This is on postgresql 11.4.

Thanks,
Luca