Pistons take 3 redstone ticks to extend and 1 tick to retract, which should both take only 1 rs tick

The screenshot shown below is a falling-edge detector which should emit 2 ticks pulse on a falling-edge and shouldn't emit a pulse on a rising edge.

The repeater is set to 2 ticks delay. On a rising edge the piston should take 1.5 redstone ticks to extend so there should be no pulse emitted from the output. On a falling edge the piston should retract instantly (0 redstone ticks) so there should be 2 ticks pulse from the output.

However in 0.15.0, pistons take 2.5 or 3 redstone ticks to extend and 1 or 1.5 redstone ticks to retract, so the said circuit instead acts as a 1 tick dual-edge detector.

According to a developer, it might be a result of the redstone circuit ticking separately from the block entities (piston). During the 2 tick: Redstone ticks first, which will propagate the repeater. The piston ticks afterwards placing the moving block as full block.