Mobs spawning on the player.

Even in lit areas at night, in a cave and especially in the nether, mobs sometimes spawn on top of you, I have been infuriated by this, one example is in the nether I was bridging up and across at a decent pace, I was pretty high up maybe, 30-40 blocks, a magma cube spawned on the 1x1 block I was standing on and it knocked me off, another example is I was in a cave and boxed myself in for safety, and lit up the area with a torch maybe a 1x2 or 2x2 area, out of nowhere a zombie spawned on my head and killed me, yet nothing was there about 15 seconds ago when I lit it up, it was when I turned around it spawned