I'm still not clear on what happened on that vote, other than that Leon voted for Rust so Poirot must have voted for Leon, but he could have voted Rust and gotten Rust out, which seems like a far better move? Maybe it was the sort of thing where he wasn't certain how the votes would fall, or that he'd irrevocably burn Rust's jury vote if he tricked him into voting Leon and then backstabbed him?