Nether mobs missing [Solved - Not a bug]

I'm currently playing a world I created in august 2012 (shortly after the release of 1.3.2 update) and I built today my first Nether portal in this world, as I was waiting for the redstone update to go in the Nether.

But as I entered the Nether I could'nt hear any mob screams, neither ghasts nor zombie pigmen. I can't tell about wither skeletons or blazes as there is no fortress next to the portal and I haven't gone far in the Nether as for now.

But anyway I should have seen at least some ghasts floating in the "sky" and heard zombie pigmen, so why are there none of them ?