Probably fallout from trying to juice the stagnating ad revenues. In a perfectly relevant search nobody would ever click on ads. I heard from Bing engineers (c

Probably fallout from trying to juice the stagnating ad revenues. In a perfectly relevant search nobody would ever click on ads. I heard from Bing engineers (circa '09-10) that every time Bing would release a relevance improvement, Google would increase their relevance metrics shortly thereafter, suggesting that their relevance is being artificially held back. It makes economic sense, and it'd be super easy to implement, so I believe it.