Forgive the redundancy; I didn't read every word of every post.
Being a chief resident depends, first and foremost, on the specialty.
In shorter residencies, like pediatrics and internal medicine, being the chief resident is a coveted position for most folks because it often indicates the highest level of confidence that a program can place in a person. These chiefs have finished their residency and are either board eligible or board certified. As such, they are qualified to attend on general medicine/pediatrics teams, etc., etc., etc. Even though these folks are delaying their training or passing up a year making the big bucks, very few turn down the opportunity. Also, many of these folks have already matched into fellowships, and the fellowships - who recognize the prestige of the position - will gladly allow them to defer for a year.
Then there are the longer residencies, like surgery, radiology, or OB/GYN. Tacking on an extra year of "residency" for these specialties is considered unbearable, so the chief resident is simply a senior level resident. Plus, these residencies are often smaller in size, so there may not be a need for a designated full-time chief. Compare that to a big IM department that may have 50 residents plus medical students. At many programs, the duties of the chief rotate throughout the year, so to not place an undue administrative burden on a single person who is still preparing for the boards. At other programs, all of the senior residents were known as chiefs; I've even know a general surgery program where all 4th and 5th year residents were called chiefs.
Clear as mud?