[GAMEPLAY] Thresh being falsely credited with kills?

Just had a game.

I was Caitlyn ADC. My support was Thresh. Enemy was Graves, with Veigar support.

Of our first 6 kills in lane, Thresh got 3 of them. The 3 I got all had something in common, Thresh was running away and not targeting the person I killed as I killed them.

The other 3, we just couldn't figure out for the life of us how Thresh got the kill... On 2 of them he was stunned by Veigar's E when I shot what would have been the killing blow, and Thresh got credit. Thresh doesn't have a DoT, he didn't have red buff, and his ignite was already on cooldown both times from an earlier attempt to kill. The third he could have actually landed the last hit, but there wasn't a visual of him using an auto attack and the enemy died as my auto attack made contact in all 3 kills.

I suppose it's possible we both had a ping spike on that kill (I had a few ping spikes that game) resulting in us not seeing it properly, but I have no idea what could have happened when he was stunned. My only thought is that there must be some sort of interaction between the champions involved that's making the client think Thresh is landing the killing blow rather than Caitlyn?