Sometimes mobs can't see you even when you're in visual range of them

Some mobs (Usually creepers, zombies and wither skeletons) will not immediatly see you and go after you even if your in visual range of them.

This might not be easy to reproduce, try looking for a creeper in a cave or a wither skeleton in a nether fortress, you may notice that even though you're in range of them they wont go after you immediatly.