Monsters Spawning In light levels of Eight on SMP.

I am building a tower on a man made island and for most of the time there are no mob spawns except for squids, Iron golems, and slimes (Theres a 2x2 chunk slime farm on the island).

Ocassionally I will spot a creeper, a skeleton, a spider, or a zombie roaming around on the ground floor. I've searched every spot for light levels of 7 and below and there are only light levels of 8 and above.