Inconsistent conversion of ADDTIME() to number

SELECT

  CAST(ADDTIME(0, 8385959) AS SIGNED) AS ci,

  CAST(ADDTIME(0, 8385959) AS DECIMAL(30,0)) AS c0,

  CAST(ADDTIME(0, 8385959) AS DECIMAL(30,1)) AS c1,

  ADDTIME(0, 8385959)+0 AS pi,

  ADDTIME(0, 8385959)+0.0 AS c1;

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

| ci   | c0      | c1        | pi      | c1        |

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

|  838 | 8385959 | 8385959.0 | 8385959 | 8385959.0 |

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