I think therein lies the conflict. Michael was sincerely sorry and repentant that he killed Fredo, but that was not the only sin he committed or confessed. He was neither sorry about his criminal activities nor any of the other men he killed or ordered to be killed. He just casually mentions these sins without a thought or hint of regret and only truly displays his guilt when confessing his killing of Fredo. I think ordering the death of his mother's son and his father's son tormented Michael his whole life, but I highly doubt he ever felt an inkling of remorse for killing Sollozo or McCluskey or any of his other sins for that matter.

If FFC and Puzo were the sticklers for detail they are given credit for, I think both DC and TB could be correct in their analysis. Michael truly does feel guilt and sorrow about his sins as DC claims, but only a select few such as Fredo's murder. Perhaps Lamberto saw through this in Michael's confession and thus never completed the confession in a traditional way by assigning penance, etc. as TB stated. Confession isn't like a plea bargain where you can whittle away certain sins that don't seem as severe as others.