Assume binomial outcomes given: right or wrong (exclusive).
So, if you are the first letter, then I am the second.
Example: RR <-- Ich and SB are both right.
Thus the outcomes are:
RR,WR,RW,WW
Since WW is 1/4 of the potential outcomes, we'd be wrong about 25% of the time. Luckily the chart helps that out and skews the stats :)