I noticed that too. I'm wondering if it came down to exact calculation in a physics step or something to determine how ma y pieces or was actually touching.
Or maybe it was the nature of box colliders vs circle colliders?
I thought about changing it, but then decided to keep the nice randomized destruction. Help keep the player on their toes. Destroying 1 block at a time would take forever, but having random huge chunks felt like a nice bonus when it boosts the time bar. :)