Why this threshold for JaroWinkler distance ? 

Hello I am currently digging in did_you_mean to understand it better. 
So I have landed on [this](https://github.com/ruby/did_you_mean/blob/ef0c37a35ae1216f3c804daf436254c7dee710bc/lib/did_you_mean/spell_checker.rb#L14). It is the threshold for selecting a word in the dictionnary and I was wondering why did you choose this particular threshold ? 