I was wondering the same and then I noticed that the members who had their details posted under their username were those with a higher ranking [there is "newbie", "member", "full member", "star member", "hero member", etc.]. From my observations, these statuses are gained through "hard work": you have to contribute to the forum [post questions, reply to questions, etc] before you can publish your details.
I'm waiting for elder members of the forum to correct me if I'm wrong, since this is only what I believe is the case.