I think there is a bug in piston mechanism.

When i'm playing game i found one weird thing. it is about piston's working mechanism. Let's see picture. At the first picture, i made some redstone circuit and the piston's state was off. Then i placed block on the piston and the piston was work like the second picture. The third picture, that i broke the block on the piston, there is a piston which state is 'on'. I feel some weird in that because first picture and third picture's shape of redstone circuit is same but piston's state is different. I don't know whether it is bug or normal phenomenon in minecraft because i'm not old player in this game, so i want some answer in this phenomenon. (I'm not good at english bcz i'm Korean so if u have some questions while reading, please comments so that i could answer.) Thank you to read this writing.