From Shannon's formula for information content[1]. The Idea is that the less likely an event is the more information it imparts.

  I = -log2(p)

In this case they used p = 1/100

1 - https://en.wikipedia.org/wiki/Information_content

Because it uniquely describes a user within a group of 100 other people. 2^(-6.6) ~ 1/100

log2(1/100)