When numerology content calls a birthday number "lucky," it's making a claim about symbolic association, not about probability. No numerology school claims to have tested whether people born on a particular day actually experience better outcomes — the label is inherited from tradition, not derived from observation.
Different traditions reach their "lucky" labels through different routes. Western numerology tends to lean on symbolic meanings built up around each digit. Other cultural traditions, like Chinese numerology, lean on how a number sounds when spoken aloud. The traditions don't always agree, which is itself a clue that "lucky" isn't describing something objective.