Also, you can request the free chick be one of the breeds you ordered or completely random. That is helpful if you need heat-hardy or cold-hardy or want to avoid feathered legs or only have feathered legs etc. Have to place order by phone for that, though.
From what I understand, at McMurray they have bins of unsold chicks, and the mystery chick comes from there. Probability is higher to get a male since more boys go unsold, but you could get a female. So technically unsexed with a higher chance of boys.