3 ideas
The winning player does not aim for the theoretically optimal contract or play. They aim for the best outcome that is actually attainable given this partner, these opponents, and the risk of misunderstanding. Chasing the ideal result with a partner who can't execute it produces worse average outcomes than settling for a safe, reachable good result.
Your bidding and play should be calibrated to your partner's real skill and habits, not to how a competent partner would behave. A sound call that a weak partner will misread is a bad call, and a technically inferior call that the partner will understand is a good one. The unit of performance is the partnership, so optimizing your own correctness while ignoring the partner's limits loses points.
The Unlucky Expert makes technically defensible plays that lose, then blames bad luck or partner rather than his own failure to adapt to the table. Because each loss can be explained as a statistical accident or someone else's error, he never updates his approach and keeps losing. Attributing bad outcomes to external factors lets a skilled person stay persistently unsuccessful.