Indeed, they'd cache the [A,B,C] case as well as the [B] one separately, which does reduce the effectivity of caching if there are many common assets and combin

Indeed, they'd cache the [A,B,C] case as well as the [B] one separately, which does reduce the effectivity of caching if there are many common assets and combinations. However in their case the set of images is (supposedly) wildly different per page, so I guess this is not much of a problem.