Am I Thinking Right Or Is It A Bug?

Please read the following statements. I got my expected results when I use the 3rd statement. How come second statement is giving me junk results.
First statement is just opposite of second and third statement,I believe.

First
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select table1.column1 from table1,table2 where table1.column1 =
table2.column1

select table1.column1 from table1 where column1 in (select column1
from table2)

Both results are same here.

The result is all the common rows.

Second
------
select table1.column1 from table1,table2 where table.column1 <> table2.column1

Third
------
select table1.column1 from table1 where column1 not in (select column1 from table2)
The result is