interesting, I did that just now (with out having seen your comment) and got 13% and 33% (for future reference this comment is being made about an hour after parent)
I tried the same guess, and felt the same confusion. Whatever quality it is that the relatedness factor measures doesn't seem to align well with my sense of word association.
Check. That worked.
"Capital" --> "Letter"
Did not work at all. And yet the two words are side-by-side with extreme frequency.
So, basically, I don't know how this game gauges relatedness. I do know that I don't like it.