Why are spiders spawning on my 2x2 platform?

The wiki says normal spiders need a 3x3x2 area to spawn, I was wondering how true this is?

I've noticed them spawn in areas 2x2x2? Any help is appreciated.

Source: http://www.minecraftwiki.net/wiki/Spider

UPDATE: Image of spiders spawning http://postimg.org/image/67bo9lkh1/

Solution 1:

The 3x3x2 spawning requirement is referring to the empty space. Spawning only requires a solid block under the spawn area.

Light level of 7 or less, 3x3x2 space on solid blocks (spawns only on top of the central block). Top half of space may be occupied by transparent blocks (glass, etc.).

This means that the requirement is a 3x3x2 area of empty space with a solid block under the middle empty block. With that in mind, it is easy to see numerous valid spawning locations in your screenshot.