I still wonder if there's some secret modifier in each match that sometimes adjusts one side's penetration. The results seem a bit skewed for random distribution.
I just did some clan wars. In the final, my TD was 8 shots, 8 hits, 8 pens.
In the land next match, my whole team said it was suddenly a disaster for penetration.
My T110E5 with gold had 11 shots, 8 hits, and 3 pens for a total of 128 damage. I think I had 7 bounces in a row, the 7th to the ammo rack area of a T62A.
Most of my shots were to the sides of enemies.
But the enemy had 5 hits and 5 pens to my tank, mostly front.