When two pistons are stacked then activated, the top piston disappears leaving the piston head

What I expected to happen was...:
The Bottom piston to put the top piston upwards, then activating the top piston

What actually happened was...:
both pistons activated at the same time but destroyed the block of the top piston

Steps to Reproduce:
1. place a Sticky Piston facing up on the ground
2. place a normal Piston on top of the Sticky Piston facing upward
3. place a repeater on the ground facing the Sticky Piston
4. place redstone from the repeater to a block
5. place a lever on the block
6. place redstone from the block from the back
7. make block-steps going 3 blocks high right against the pistons
8. lead the redstone coming from the back of the lever block up the stairs but not on the step
9. flip the lever to ON position