Ed?: This looks to be white's strongest attack at the top gaining a ko for the group. could be at 'a' but may be safer here. White has a lot of ko threats. If at 'b' white wins by playing at .
tderz: Ed, I think you're very strong in problem-solving.
I guess the players did not see the ko.
Ed?: Black seems to have more than enough liberties to finish white so perhaps I'm missing something.
tderz: ... or perhaps they (white) saw the ko and decided >that she needed definitively to kill the black group.
Indeed the players proceeded as you suggested in your last diagram.