In the first round when Usogui lose, Shion stated that Usogui make a mistake and then follow up by apologizing to leader. He presses the button after hesitating for a few second.
In the second round, Lalo lose, so Shion who don't give a shit about Lalo immediately announce the result and press the button.
In the third round, Usogui lose. Shion once again hesitated, and causes an inconsistent timing when announcing the result.
Let's remember Usogui's question. "Who makes the poker hand? Ref or machine?".
If a ref makes it, then there should be no room for error and inconsistent announcement timing shouldn't have happened. If a machine makes it, then the same could be said as previous case. Kakerou is peerless in terms of technical issues.
So the answer is a Human, but who? Why would Kakerou ref trust someone else to do it?
If we link this question with the inconsistent announcement timing, Usogui can assume that Something is making even a Kakerou referee hesitate and is related to someone who is making the poker hands and guess what can make the infamously impartial referee drop a sweat and hesitate? Leader.
We will see Usogui's train of thought like this:
"Why would leader make an impact to this game?" Because he's participating in this game.
"He's in the ban as my ally, so he can only participate as a player." Putting two and two together Usogui can assume Hal is the one making poker hands and is a player as his ally!
Kakerou is pictured as flawless but time and time again we see that they are humans who has some attachment to ofhers too. This fact was sidelined by the fact that Shion and Takumi were seen to not have a problem bringing such a dangerous game for Leader to participate, that is until Shion finally show his weakness in the game. Takumi has just recently joined so he has no attachment to Leader, but Shion is definitely different.