Different result when group by with correct function

create table t (c int);

insert into t values (20121212),(9),(19800101),(20121212),(19800101);

select c, count(*) from t group by  c desc with rollup; 

MariaDB [test]> select c, count(*) from t group by c desc with rollup; 

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

| c              | count(*) |

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

| 20121212 |        2 |

| 19800101 |        2 |

|        9           |        1 |

|     NULL     |        5 |

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

4 rows in set (0.001 sec)

select c, count(*) from t group by convert(c, int) desc with rollup; 

MariaDB [test]> select c, count(*) from t group by convert(c,int) desc with rollup; 

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

| c              | count(*) |

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

| 20121212 |        2 |

| 19800101 |        2 |

|        9           |        1 |

|        9           |        5 |

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