Why not exists and minus give different result

Question

HI,
I am writing thes query by using two different operators they should give me the same output but they dont. Please can anybody explain why they give different output.
query 1
SQL> SELECT COUNT(*) FROM
2 (select orig_idx
3 from rel15_au_poi a
4 MINUS
5 SELECT ORIG_IDX
6 FROM REL14_1_AU_POI_INTERM B) ;
COUNT(*)
244312
query 2
SQL> select count(*)
2 from rel15_au_poi a
3 where not exists
4 (select null from rel14_1_au_poi_interm b where a.orig_idx=b.ORIG_IDX) ;
COUNT(*)
245341
best Regards,

Answer 1

Because you have duplicates in rel15_au_poi which are counted with exists and removed with minus.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/queries004.htm#sthref3147
MINUS Example
The following statement combines results with the MINUS operator, which returns only unique rows returned by the first query but not by the second:
SELECT product_id FROM inventories
MINUS
SELECT product_id FROM order_items;