spiders spawn on less than a 2x2 area

I made a wizards tower with the flat sides 5 long; inside the floor is every other block is a hole

What I expected to happen was...:
Describe what you thought should happen here
only zombies skeletons and creepers spawn
What actually happened was...:
Describe what happened here
every hostile mob spawns
Steps to Reproduce:
1. build a wizards tower using this link (click on wizards tower change everything but the side tower on 12) http://www.plotz.co.uk/plotz.php
2. build it, put water flowing towards center on bottom floor.
3. watch the spawns may take a little while