In the early version of the script Tom says to Mike:
"The Senator can be set up; but he thinks of himself as a clean politician. So it's got to be on terms he can live with: campaign contribution, donation to a charitable cause that he controls, things like that. If he gets even the inkling that you think you're buying him, he'll freeze up. Nevada's a funny state, they like things both ways here... All right. Turnbull says the Senator will be here at two-thirty, and he's been primed. He knows you'll want to meet with him alone, and he knows it's about the Tropicana's license. At any rate, he expects to be introduced around to some of the influential people here today, and generally treated as an ordinary guest. Just go light on him, Mikey, sometimes the biggest crooks don't like to think of themselves as crooks..."

***
So, in the conversation with Mike, Geary doesn't seem to care whether he's seen as a but he thinks of himself as "a clean politician". He openly wants to squeeze the family. He doesn't want the whole thing to be covered like a "donation to a charitable cause that he controls". He insults Michael's family and all Italians. So, the wrong information was mainly about the character of the senator. Michael was prepared for an easy game he had to play with Geary. He had to change the game, the way of playing, the moves.
That's why he gives a cold stare to Tom.


(Well, I know: The film is the film, the novel is the novel, the scripts are the scripts. But certainly we have to play little games...)