It didn’t know the correct response or was not confident enough to answer 16% of the time (5 of 31). Nobody at all could give an answer to 6% of all questions (2 of 31).
If the two questions we know nobody could answer are excluded, Watson knew the right answer 90% of the time (26 of 29).
Do individual Jeopardy contestants correctly know more than 84% of the answers? Two questions were left unanswered correctly by everyone, 93% (29 of 31) is already the upper bound for the human performance in this round, that’s not very far from 84%.
Looking at the six rounds of Jeopardy the J! Archive has of games with Ken Jennings and Brad Rutter [0] we see that the average upper bound in those six rounds was 87% or 26 questions (of 30). The minimum was 23 and the maximum 28 (of 30). That’s, again, very much an upper bound.
Looking at all this, I’m pretty confident that Watson would be doing well even if it had human reaction times. (This is how I would change the game: Within 200ms or so – whatever the human reaction time on a Jeopardy buzzer is — after the buzzers are open, the player to answer is randomly selected from all who managed to buzz in. After those 200ms everything stays the same. Players are also not punished for buzzing in too early. This is a minimal change to the game, not a completely different game which I think is important.)
During tournaments of champions, Jeopardy! is not about how many correct responses you can come up with, as all competitors will know the vast majority of them. It's all about timing. I don't know the exact statistics, but it seemed like Watson knew about 75% of the correct responses. I strongly suspect the two human contestants knew a greater percentage. On day 2, it just came down to timing. Watson was only beaten to the buzzer three times when it knew the correct response.