Pistons with a powered block two blocks above them, with any block in between including air, are considered powered and will extend if updated.

Steps to reproduce:
1) place either type of piston
2) place any block on top of the piston
3)place a redstone block on top of that, or place any solid block and power it with a lever
4)remove the block placed in step (2). this should update the piston and cause it to extend. any block can be placed on top of the piston, and it will remain powered.