I was surprised, because it seems counterintuitive, but the math (below the link) is irrefutable.
Think about it first before peeking. Why would aces over 8s be better to have than aces over kings?
Aces over kings would beat any other full house, but isn't the best for beating straight flushes. From the explanation at Futility Closet:
Specifically, each of the hands AAA99, AAA88, AAA77, and AAA66 will prevent 13 straight flushes — each ace prevents one, and each spot card prevents five.
These are thus the best to choose; of the 40 possible straight flushes, each leaves only 40 – 13 = 27 straight flushes that can beat us. By contrast, the impressive-looking AAAKK leaves 40 – 6 = 34, or even more if we haven’t covered all four suits.