Subquery with IN or EXISTS

Hello,

does anybody know why this:
SELECT * FROM tab WHERE col1 IN (SELECT col2 FROM TAB2)

equals this:
SELECT * FROM tab WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 =
col2)

but this:
SELECT * FROM tab WHERE col1 IN (SELECT col2 FROM TAB2 WHERE
col3='huu')

equals _NOT_ this:
SELECT * FROM tab WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 =
col2 AND col3='huu')

E.g. resultset is not the same in last two statements.
Can I get same set as IN statement somehow using EXISTS (performance
issue)?