What I think is happening is that the piston is being powered by sheer magic, the video will explain!
www.youtube.com/watch?v=GQK2yIsB28k&feature=youtu.be
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qmagnet
migrated
Alright, thanks! - I wasn't going crazy then.
qmagnet
Nope. You're not crazy. It really shouldn't exist but youtubers made it famous and Mojang has kept it in. It can be tricky figuring out a way to power around it. Good luck.
migrated
I'll end up using the bottom piston elsewhere to block the path of the cave spiders. Thank you for your comment!
This is famously coined as a BUD or Block Update Detector
http://minecraft.gamepedia.com/Tutorials/Block_update_detector#Piston_Based
The "glitch" is so widely accepted and exploited in the Minecraft community, I think there would be an uproar if Mojang actually fixed it. The BUD occurs when you power a signal from above the piston.
See MC-108 for full information.