Two hospitals report surgical success rates. Hospital A: 85% overall (850/1,000 patients). Hospital B: 80% overall (800/1,000). However, stratified by procedure risk:

| Risk Level | Hospital A | Hospital B | |---|---|---| | Low-risk | 90% (450/500) | 95% (380/400) | | High-risk | 70% (400/500) | 60% (420/700) |

Hospital A appears better overall but is worse in BOTH risk groups. Why does this paradoxical pattern occur?