Subselect with incorrect column not a syntax error?

Could someone please explain to me why the following select does not
result in a syntax error? (9.0.3)

begin;

create table x( c1 integer , c2 integer);
create table y( c3 integer, c4 integer);

select * from x where c2 in ( select c2 from y where c4 = 2 );

rollback;

Mike