Birthing Hulk and Mirrorpool

Barrelrolla

If there is a Mirrorpool on the battlefield and Birthing Hulk enters, he doesn't activate it, but when the Eldrazi Scion enters, it activates it, so instead of my Birthing Hulk becoming 10/8, my Scion becomes 3/3, which is lame, unexpected and totally not how it should work, because a creature already entered, but did not trigger Mirrorpool for some reason.