page 48. The last two sentences.

[yongduek]

The book says

" ... so relative to the true answer the median was an over estimate by around 10%, and only around 1 in 10 people got that close."

The median is about the number of jelly beans, not the number of people. The number of people who guessed within the range of true value plus/minus 10% will be a lot more than 1 in 10.