Creepers explode on other side of walls

I was on one side of a two block tall wall of cobblestone blocks and a creeper was on the other. There was no way for him to get to me. I jumped and hit him with my sword. The creeper looked up and started to explode. I waited for him to stop, then jumped and hit him again. This time the creeper did not stop his countdown when I landed and exploded on the other side of the wall. It had no line of sight to me at all. This has happened twice.