Something Sano does makes Saitou wonder briefly, but then he gets it.
Sano did stupid shit as a child. Turns out maybe Saitou did too.
With the world falling to pieces around them, the truth comes out.
Saitou is very irritated, for some reason, by the new development between Kenshin and Sano.
Sano has THAT effect on Saitou just by lying around asleep.