Turnbull is one of our most knowledgable posters. He said itmuch more eloquently than what I'm about to write, but this is the gist of it:

There is a recurring theme of characters underestimating one another throughout the first two GF films:

Vito underestimated Sollozzo

Sollozzo & McCluskey underestimated Michael

Hagen underestimated Barzini which lead to Sonny's death

The other Families underestimated Vito at the sitdown

Carlo underestimated his own safety in thinking that Vito & Michael would let him off the hook

And Roth & Michael both underestimated each other in GFII