Tom was a lawyer, and he approached things as a lawyer would, notwithstanding his upbringing in a Sicilian household. But he was not a Sicilian, and did not have the famous "Sicilian cunning" that Puzo referred to in the novel. The most revealing example, IMO, is his failure to anticipate that Carlo would want revenge after Sonny publicly beat and humiliated him. From Tom's lawyerly viewpoint, Carlo--who depended on the Corleones for a living--would never bite the hand that fed him. And he'd never escape detection. But a Sicilian would anticpate Carlo's overarching need for personal revenge, no matter how illogical or dangerous.
In the novel, following Sonny's murder, Tom admits to himself that he's not a wartime consigliere: "Old Genco would have smelled a rat." Exactly!