Splash damage

I don't get it, I built a split rail fence and makeshift fences starting at the very next square, to test the radius of splash damage.
Left it there for over a week and more, then destroyed the split rail, no damage done to the makeshift fences... what am i missing ?
Left it there for over a week and more, then destroyed the split rail, no damage done to the makeshift fences... what am i missing ?
