Spiders climb invisible bedrock on finite maps

While I was playing on my singleplayer creative world, I decided to go bug hunting. So I spawned in some spiders. When i hit a couple, then ran to me, trying to attack, as normal. But, when some of them stopped and walked away, they hit the invisible bedrock. Normal. But when I looked up a little, they were climbing the invisible bedrock! This was supposed to be for the Pocket edition, but my iPad decided to be a jerk, so I can't change the version. Sorry about that.