Warning generation is inconsistent when converting from number to string

mysql> select cast('c1' as signed) as c1,cast(concat('c2') as signed)as c2,cast('c3' as double)as c3,cast(concat('c4') as double) as c4, cast('c5' as decimal) as c5,cast(concat('c6') as decimal) as c6; show warnings;

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

| c1 | c2 | c3   | c4   | c5 | c6 |

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

|  0 |  0 |    0 |    0 |  0 |  0 |

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

1 row in set, 5 warnings (0.00 sec)

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

| Level   | Code | Message                                                 |

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

| Warning | 1292 | Truncated incorrect INTEGER value: 'c1'                 |

| Warning | 1292 | Truncated incorrect INTEGER value: 'c2'                 |

| Warning | 1292 | Truncated incorrect DOUBLE value: 'c3'                  |

| Warning | 1292 | Truncated incorrect DECIMAL value: 'c5'                 |

| Warning | 1918 | Encountered illegal value '' when converting to DECIMAL |

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

5 rows in set (0.00 sec)