"Well," I said, "Brett and Mike ought to get in to-night."
"I'm not sure they'll come," Cohn said.
"Why not?" Bill said. "Of course they'll come."
"They're always late," I said.
"I rather think they're not coming," Robert Cohn said.
He said it with an air of superior knowledge that irritated both of us.
"I'll bet you fifty pesetas they're here to-night," Bill said. He always bets when he is angered, and so he usually bets foolishly.
"I'll take it," Cohn said. "Good. You remember it, Jake. Fifty pesetas."
In the passage above, from chapter X, why does Robert say “I rather think they’re not coming?”