We had the same experience as JHS with my kids cohort of friends. The kids who were legacies at Harvard or Yale and applied to both got into both the school where they were a legacy and the other school. The legacies were all in the top 2% of the class with high SAT scores.
The math is not that difficult. If you have 100 identical kid and 10 of them are legacies. If you accept 3 legacies and 3 other kids. The overall acceptance rate is 6%, and the legacy acceptance rate is 30% even though they all have identical stats.