A video is attached to this bug report that shows the issue clearly (sorry for the bad quality).

Anyhow, if a piston is placed parallel to a redstone (redstone A) that is connected to another redstone above an adjacent block (redstone B), the piston will only be powered when redstone B is placed on a block that doesn't conduct redstone power, such as a slab, glowstone or even glass. I don't know if this behaviour is just working as intended, however, I'd appreciate some clarification. Thanks!