Wrong result with GROUP BY ... WITH ROLLUP

CREATE TABLE t(a int,b int,c int, amount1 int, amount2 int);

INSERT INTO t (a, b, c, amount1, amount2) values (1,1,1,1,1);

INSERT INTO t (a, b, c, amount1, amount2) values (2,2,2,2,1);

INSERT INTO t (a, b, c, amount1, amount2) values (2,3,3,3,1);

SELECT *,

    IF(`t_reorder`.`b` IS NULL, @ccount1:=amount1, @ccount1) AS `a_amount1`,

    IF(`t_reorder`.`b` IS NULL, @dcount1:=amount2, @dcount1) AS `a_amount2`

FROM (

	SELECT *

		FROM (

  		SELECT `a`, `b`,`c`,

			SUM(`amount1`) AS `amount1`, SUM(`amount2`) AS `amount2`

		FROM `t`

		WHERE (c IN (1,2,3)) AND ((`a` IS NOT NULL) AND (`b` IS NOT NULL))

		GROUP BY `a`, `b` WITH ROLLUP) `t_rollup`

	ORDER BY `a`, `b`

)`t_reorder`;

mysql.exe  Ver 14.14 Distrib 5.5.28, for Win32 (x86) 

(Correct)

+------+------+------+---------+---------+-----------+-----------+

| a    | b    | c    | amount1 | amount2 | a_amount1 | a_amount2 |

+------+------+------+---------+---------+-----------+-----------+

| NULL | NULL |    3 |       6 |       3 |         6 |         3 |

|    1 | NULL |    1 |       1 |       1 |         1 |         1 |

|    1 |    1 |    1 |       1 |       1 |         1 |         1 |

|    2 | NULL |    3 |       5 |       2 |         5 |         2 |

|    2 |    2 |    2 |       2 |       1 |         5 |         2 |

|    2 |    3 |    3 |       3 |       1 |         5 |         2 |

+------+------+------+---------+---------+-----------+-----------+

mysql.exe  Ver 15.1 Distrib 5.5.28-MariaDB, for Win32 (x86)

(Wrong values on a_amount1 and a_amount2)

ORDER BY `a`, `b` does not have any effect to this qyery

+------+------+------+---------+---------+-----------+-----------+

| a    | b    | c    | amount1 | amount2 | a_amount1 | a_amount2 |

+------+------+------+---------+---------+-----------+-----------+

|    1 |    1 |    1 |       1 |       1 |         6 |         3 |

|    1 | NULL |    1 |       1 |       1 |         1 |         1 |

|    2 |    2 |    2 |       2 |       1 |         1 |         1 |

|    2 |    3 |    3 |       3 |       1 |         1 |         1 |

|    2 | NULL |    3 |       5 |       2 |         5 |         2 |

| NULL | NULL |    3 |       6 |       3 |         6 |         3 |

+------+------+------+---------+---------+-----------+-----------+