With the most tantalizing mystery in recent political history solved, the cottage industry of amateur detectives devoted to uncloaking Deep Throat has turned to confront one of the last remaining questions: How did so many people guess so wildly wrong for such a long time? W. Mark Felt... who revealed himself on Tuesday as The Washington Post's secret source on the Watergate scandal, usually fell low on the list of suspects, if he was there at all, in scores of books and articles speculating on the subject.... Yet even now, some skeptics are unconvinced Mr. Felt is the Deep Throat...