My guess is that Clemenza had been suspected of being involved with Paulie's betrayal, and that maybe he felt he would be on the outs with Michael once he took over. Tom also may have been less confident in Michael's chacs of success, because Michael's rejoinder is a real show of confidence. He says this betrayal was a "smart move," and Tessio was always smarter. In other words, Michael's plan to have everyone underestimate him was working so well he exposed the traitor in the Family about whom his faher warned him.