In what way does benvolio story differ from the actual sequence of events

I believe the answer is: Benvolio claimed that Tybalt automatically killed Mercutio and that Benvolio interfered.

However, Benvolio statement never happened. From his actuions, the audiences would most likely think that the lie that Benvolio said is his personal attempt to protect both Romeo and himself. But Benvolio still fail to convinved Lady Capulet