Rebirth passives

There's three in the game, Aatrox, Anivia, and Zac.

Aatrox- Automatically revives, health based on his resource. 225 second cd.

Anivia- Turns into an egg for 6 seconds, if she's alive at the end of the egg, she gains health equal to how much the egg had. Receives -40 to +20 armor and MR depending on her level for the duration. 240 second cd.

Zac- Explodes into four blobs each with 12% of his max health and 50% of his armor/MR, if any of them survive for 8 seconds, Zac revives with 10-50% of his health depending on the blobs remaining. 300 second cd.

Why is Aatrox's on the lowest CD when it's the only one that revives him no matter what? Why is Zac's on the highest CD when his is the hardest to pull off?