Some passive mobs that do not naturally spawn or despawn are still included in the passive mob cap

On the debug screen, the C value under the mob caps counts sniffers, camels, and allays, despite none of these naturally spawning or despawning.
Meanwhile, iron golems, villagers, and snow golems do not.

Camels and allays generate as part of a structure but are not part of the passive mob spawn cycle, so there is no reason for them to be included in the mob cap.

 

I tested it in a superflat creative world by using /kill @e[type=!player] and then spawning a bunch of sniffers. No new passive mobs spawned.