If a piston is unable to push but is still powered, the piston doesn't remember that it is being 'powered'. so when an additional redstone signal is given through a block (i.e. another redstone line powers the piston through a block) the piston will update itself, despite the fact the the piston was already 'powered' (even though it was unable to push).

How To Replicate:

Place a block with two levers attached. Place a piston next to that block facing away. Place another block in front of the piston. Lastly, place a furnace in front of the last block and THEN flick one of the levers.

Your setup should look like this:

Destroy the furnace. The piston should stay retracted.

What i expected: flicking the other lever should NOT update the piston as it is 2 blocks away and the piston is already powered by the first lever.

What happened: flicking the other lever results in the piston updating and extending.

The piston should still be in a 'powered' state, but yet it still re-powers. even whan that source is from 2 blocks away.