Incorrect results with NOT IN

[Nick <nboutelier@xxxxxxxxxxx>]

I have a weird scenario on a table when I run this query...

table1 has 1500 rows
table2 has 1200 rows
table2.id is a foreign key of table1.id

SELECT COUNT(*) FROM table1
WHERE id NOT IN (
  SELECT id FROM table2
);

however, using NOT EXISTS works

SELECT COUNT(*) FROM table1
WHERE NOT EXISTS (
  SELECT id FROM table2 WHERE table1.id = table2.id
);

Can you not use NOT IN over a specific number of values??