Double piston extenders don't work with more than one adjacent piston

Steps to Reproduce:

1. Build the design for a double piston extender as shown in the photo
2. Build another piston beside the original two pistons, connecting redstone to the other pistons.
3. Place a lever where it is designated in the photo and activate it, and then deactivate it.

Observed Result:

One block is always higher than the other

Expected Result: The block should both be at the same height when they are retracted