There are a few entities that spawn in regardless (Or perhaps they are generated and left around spawn not sure?) But for what I can tell if you start a world with Mob Spawning turned off you can still capture Mob Data into a Storage for a slight moment before they despawn

Is this intentional? 

I attached a structure that takes any entity and stores them in a storage before doing a few other commands, you can see Rabbits, Bees, Cows, Pigs, etc... Spawning in and then respawning moments later (However we can still access them via selectors during those moments)

(This probably effects older versions but only testing in snapshots currently)