While I, too, found that part intriguing, the article mentions that the guy has been working as a prison guard since being rejected. Could be that he just wanted to make money by being a police officer and that department missed out on a great hire. Not to mention the fact that the nature of the work can definitely change as long as they're promoting from within.
They seem to seek a "sweetspot" where they're smart enough to be valuable additions to the force and not be easily outsmarted by a average people but not so smart they grow bored and quit thus costing the dept time and money.