Can someone explain why auto attacks do less damage to towers overtime?

So, I was playing a game with Yasuo today, and I noticed something strange. I was attacking the turret (with plates still on), but as I continued to attack it, my basic attacks were actually doing less damage overtime. And the weirdest thing is after a plate fell off my attacks did even less damage. You'd think that after a huge armor plate falls off a turret you'd be doing more damage to it, but is seems the opposite is true!