The Mean Is Lying to You: Benchmarks Hide the Variance That Breaks Prod

TL;DR— Benchmark scores report central tendency over a fixed, static distribution of test items, but production reliability is governed by tail behavior on a shifting distribution of real inputs. A model can post a great average and still fail unpredictably on the exact slice of traffic your product depends on. Teams that only track leaderboard deltas are optimizing the wrong statistic. A benchmark score is a mean. That sentence sounds obvious, but almost nobody treats it that way. Teams read