(In Florida) There was an opening for a manager's position in my company, I know that about 6 people applied for the position, of those 6 only 3 were interviewed and eventually one of them was given the promotion. I worked for the same company in a different state (MI) and I know we HAD to interview everyone who applied for the position. Can somebody shed some light on this... I'm a little confused.

It is not necessary to interview all applicants unless there is a concern that the selection of the eventual job holder may lead to claims of discrimination. Of course, a company may have its own policies requiring interviewing of applicants.