checking a probability in Super System II

[Roy__Batty]

hello,
i'm reading the nlhe part and i came to that probability that doyle gave:
assume you have KK and another player has 2 unpaired cards lower than kings. Doyle says an ace will flop about 18% of the time.
Well, i have no idea how he comes to that result. I get something around 22% and i don't understand how the fact that my opponent's cards are unpaired can be a factor here.
Anybody knows?