Can't understand how this group by clause works

Question

The Schema is as below: (http://www.psoug.org/reference/rollup.html)
CREATE TABLE grp_rep (
person_id NUMBER(3),
division VARCHAR2(3),
commission NUMBER(5));
INSERT INTO grp_rep VALUES (1,'SAM',1000);
INSERT INTO grp_rep VALUES (2,'EUR',1200);
INSERT INTO grp_rep VALUES (1,'EUR',1450);
INSERT INTO grp_rep VALUES (1,'EUR',700);
INSERT INTO grp_rep VALUES (2,'SEA',1000);
INSERT INTO grp_rep VALUES (2,'SEA',2000);
INSERT INTO grp_rep VALUES (1,'EUR',800);
COMMIT;
Query1:
SELECT person_id, division, SUM(commission)
FROM grp_rep
GROUP BY person_id, ROLLUP (person_id, division);
Query2:
SELECT person_id, division, SUM(commission)
FROM grp_rep
GROUP BY division, ROLLUP (person_id, division);
The results of query1 are okay. It has results from rollup and group by person_id.
But, in Query2 results of rollup are missing and results of group by division is there.
Anyone can explain how the group by clause works when there are multiple columns involving CUBE, ROLLUP and simple column names?
Regards.

Answer 1

Thank you shoblock!
but, What i m really looking for is,
How group by clause works when i add regular column along with RollUp in group by clause?
I have understood simple group by clause like
...group by column1,column2,column3....
n I also know simple rollup clauses like
...group by rollup(column1,column2,column3,...)
But my Problem is how does this work:
...group by column2,rollup(column1,column2,...)
...group by column1,rollup(column1,column2,...)
See below Results:
Query1:
SELECT person_id, division, SUM(commission)
FROM grp_rep
GROUP BY person_id,ROLLUP ( person_id, division );
Result:
PERSON_ID DIVISION SUM(COMMISSION)
     1      EUR      2950
     1     SAM      1000
     2      EUR      1200
     2      SEA      3000
     1           3950
     2           4200
     1           3950
     2           4200
SELECT person_id, division, SUM(commission)
FROM grp_rep
GROUP BY division,ROLLUP ( person_id, division );
Query2:
SELECT person_id, division, SUM(commission)
FROM grp_rep
GROUP BY division,ROLLUP ( person_id, division );
Result:
PERSON_ID DIVISION SUM(COMMISSION)
1 EUR 2950
2 EUR 1200
     1 SAM      1000
2 SEA 3000
1 EUR 2950
2 EUR 1200
1 SAM 1000
2 SEA 3000
EUR 4150
SAM 1000
SEA 3000
guys, help me make understand above results!
Regards.