Different field types for equivalent NULLIF and IF vs CASE with AVG(int_field) and 0e0 arguments

DROP TABLE IF EXISTS t1,t2;

CREATE TABLE t1 (c1 INT DEFAULT NULL);

INSERT INTO t1 (c1) VALUES (1),(2),(3),(4);

CREATE TABLE t2

SELECT

  NULLIF(AVG(c1),0e0) AS c1,

  IF(AVG(c1)=0e0,NULL,AVG(c1)) AS c2,

  CASE WHEN AVG(c1)=0e0 THEN 0e0 ELSE AVG(c1) END AS c3

FROM t1;

SHOW CREATE TABLE t2;

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

| Table | Create Table                                                                                                                                                |

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

| t2    | CREATE TABLE `t2` (

  `c1` decimal(14,4) DEFAULT NULL,

  `c2` decimal(14,4) DEFAULT NULL,

  `c3` double DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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