Question from Harrington's Book re: Breaking Down Hand Possibilities

[Joeflex]

Harrington goes into his a very detailed hand analysis beginning on P. 130. In this hand, Hero has AA and is heads-up and the flop comes 952(r) and leads and is raised all-in.

We must come up with Opponent's possible hands. Harrington says that this wager is either a bluff, or 1) Pocket Pair lower than A's (KK-TT), 2) or a set of 9's or 5's. Harrington says this is the toughest decision in poker.

Harrington says that the pair is 4 times more likely than the set (and explains the math which I'm not questioning) and also says that based on the way this hand was played, a pair is more likely.

Here's where I'm thrown way off. He then comes up with 50% pair/40% set/(10% bluff). Not sure why this is the case based on that the pair is 4x more likely mathematically and more likely based on the play, but he says these percentages are taking this into account.

Any thoughts here? This is intimidating stuff.

Joeflex