What I expect to happen:
I expect the piston to never extend just from having redstone running through it.

What actually happens:
The piston extends under certain circumstances. (see screenshots)

How to recreate:
Build the devices shown in the screenshots.

This is true with sticky pistons as well. This is not directional. (In terms of north, west, etc)

Quite quirky behavior as you can see in the screenshots.
Personally, I believe in all of these situations, the piston should extend.

I'm not going to even try to explain all of these. Just look at the screenshots.