Spawning in the Ender Dragon with

/summon ender_dragon

seems fine since it returns

Summoned new Ender Dragon

However, when killing the ender dragon using

/kill @e[type=ender_dragon]

instead of returning with "Killed Ender Dragon", returns with;

Killed 9 entities

I understand that the model of the ender dragon is very complicated, however I don't know why it chooses to kill 9 entities when there is only 1 ender dragon summoned in.