SQL & PL/SQL :: How Exactly Oracle Updates Rows Of A Table

Check the below simple sql stmts:

create table tab_tab(col1 number, col2 number);
insert into tab_tab values(1, 1);
insert into tab_tab values(2, 2);
insert into tab_tab values(3, null);
[code]......

My question is why the table content didn't look like:
col1 col2
----------
1 2
2 2
3 2
4 3
5 4

I thought that when oracle will update 1st, 2nd & 3rd rows then the sub query will get a count equal to 2, but the time when it goes to update the 4th & 5th row the sub query should get back a count equal to 3 & 4 resp. BUT that is not what exactly is happening!!