Maybe for some things, but not necessarily for everything amazon cares about, such as performance and reliability. For both of those amazon doesn't really care

Maybe for some things, but not necessarily for everything amazon cares about, such as performance and reliability. For both of those amazon doesn't really care about averages, they care about the 3 9's experience. If there's a 0.5% chance that page load will take longer than some small number of milliseconds then that's not good enough for amazon and they'll go back to the drawing board. Factor in bugs in the new layout being fixed during the A/B test (necessitating resetting statistics) and it's easy to see how it could take a year to fully roll out a big change given amazon's cautiousness.