Cho'gath Scream Hitting Outside of Hitbox

So while I was playing Nasus in the top lane a little while ago, I was up against a Cho'gath. Nothing major, trading here and there, y'know the usual.

However, there is something I noticed when we were trading. Cho's silencing scream kept hitting/silencing me when I was outside of his ability's designated hitbox. I noticed that when this happened, I wasn't on top of him or in front of him directly. I was standing off on the side, moving away when he reared back for the ability, and knew that I was out of range after the two steps it took for me to move. This is merely moments before his ability actually goes into the animation and goes off. Again, I am now no where near Cho at this point because, again, I moved the two inches to the left to side step it.

Now, where I'm standing before I move is just on the edge of the smallest portion of his ability, yet again to give me that time to move when he's ready to use it. I sent a message over all chat, mentioning how I'm surprised I keep getting hit when I'm out of his range. The Cho replied to me, in all chat, that he too was surprised they were even hitting and didn't know how it was possible.

This is a big issue if you're trying to lane around him and he can hit you without even actually hitting you.