Too Many Hostile Mobs on Well Lit Area

As you can see in this screenshot, two skeletons spawned on the roof of my XP farm, the roof is completely lit up with torches, this is the case for every lit or unlit area in this world, i don't have this issue in other worlds I play in, please help me fix this, i don't want to delete this world. 
Note -  when i first saw them they were near the torches, when i took this picture they walked away a bit