Ok. So I've done a nights worth of testing, and I think it's pretty safe to say that a certain amount of animals not spawning in peaceful.
If you would know me, you know I am a pussy in junk jack, so I put on peaceful. I notice these mobs missing:
Snails
Cows
Chickens
Birds
And a few more
Anyway, if you say this isn't true, do some testing yourself. (Not to sound rude

)