armor stand in glass

What I expected to happen was...:
it will push the armor stand in the block behind

What actually happened was...:
the armor stand got in the glass block

Steps to Reproduce:
1. place 2 blocks facing up
2. place a armor stand in front of the block
3. place 2 glass facing up in front of the armor stand
4. place 2 piston's facing at the glass blocks
5. place 2 redstone blocks behind the pistons