When you power a piston facing upwards and it pushes a redstone block, even after the redstone signal has gone away, the piston stays extended. Meanwhile, if you push a redstone block horizontally, the piston retracts once the power source has gone away. The same is true for sticky pistons

To reproduce:
1) Place a piston (regular or sticky) facing up
2) Place a redstone block on the piston head so that the piston will push it upwards when it's powered
3) Power the piston
4) Unpower the piston.
Even when you unpower the piston, the piston remains activated. This is obviously not the case with most blocks. I have only encountered this with redstone blocks.