And how does SO decide which username to put there?
It means 40% of the text was written by Mike Duncan. The user name with the most % is placed there.
Jeff describes it on his blog:
I simply sum the total size of all line contributions (insertions or deletions) from any given author in a revision, with a small bonus multiplier of 2x for the original author. We report the highest percentage of authorship in the final revision.
After so many edits, the % starts to reflect who "owns" the most content on the post.
The percentage is to show that certain a person have contributed the most to this post, and how much he contributed. That allows you to quickly tell who put the most effort into the post. Sort of a way to honor that person for writing/organizing information.