Custom Spawner doesn't spawn 2 blocks above spawner

No matter how big the mob spawning radius is, mobs will never be spawned 2 blocks above the spawner (so they can spawn ontop of spawner, but no higher)
I couldn't find this being intended behaviour, so my apologies if it is.
Haven't tested yet if it also happens with naturally created spawners or older version of Minecraft (though I expect the answer is yes).