if you create a spawner which spawns a mob with custom data, it will sometimes just spawn regular mobs around it, mostly on startup or when created if you use spawner to spawn the spawner!
EXAMPLE: Spawner should spawn a wither skeleton with armor on x 0, y 84, z 0, but it spawns regular skeletons around it before it can spawn the wither skeleton on x 0, y 84, z 0! I made some tests and realized, that the spawner will just show up the wither skeleton with armor after it spawned at least 1 skeleton! Until then is shows up a regular skeleton inside the spawner! So probably spawner containing zombies and skeletons will just load the spawner properties AFTER spawning at least 1 zombie or skeleton!