Except we have her own words in confessional: Julie: “This is a game turner right here, this vote. So, it’s a lot of pressure. I have thought it out both ways. Should I go with Russell? Should I trust him because I know how good he is? ”
She says she thought of it both ways. That is not up for us to interpret. She also knows that Russell is good to get to the end. So, she wasn't immediately thinking of fooling him.
The one question I would ask her if it was Russell's order to vote out Ralph that made her change her mind. Had he asked for her opinion, maybe that would have worked.