Is Thresh's ult working as intended (or at least as described)? [In Game/Gameplay]

I wasn't sure if I wanted to put this in bugs or not, but I encountered a sort of odd thing happening when I played Thresh on Howling Abyss earlier.

For fun, I built AP since his ult scales 1:1 and I figured I could throw it in front of people on close quarters or hook them through it and score easy kills.

However, I noticed that sometimes they'd walk through the first wall (as in, all 5 walls would still be intact) and they'd take damage, but very, very little damage.

My ult had a total damage of over 1100 at level 3, along with a void staff and abyssal scepter. On low-defense AD champs like Lucian and Varus (both of whom had less than 60 mdef), I'd expect at least 1/3rd of their health to instantly melt away after hitting it. And this is what happened. Sometimes.

Then there were a few times where I'd use The Box, and someone would walk through it, and they'd take damage, but it would be more in the order of 100-300 total. Even if I had no magic pen at all (which I had plenty of), these people should have been taking over 50% of the damage of The Box, which would have been more in the area of 500-600. This clearly wasn't the case.

I also noted that hooking people into it always seemed to proc full damage. It was only sometimes when champions walked from the outside in that they'd take nerfed damage. And sometimes they'd take completely full damage. I had no way of telling.

Their team had no champions that could shield or provide any sort of temporary protection against that sort of damage.

Again, I'm absolutely sure the walls were unbroken, because damage was dealt to the champion who broke the wall.