... one must assume that the quality and/or value of my output must be exponentially higher than sebbery's - because the labour market wouldn't sustain paying someone more than someone else, all things being equal.
But all things aren't equal. As a public servant, I am protected from labour market competition because government is a monopoly provider of services which taxpayers must purchase whether they consume them or not. Furthermore, my union has a great advantage because it negotiates with people who not only depend on my fellow members for votes, but are also spending other peoples' money rather than their own. Of course, one day the market WILL have its way (a la Greece), but for now there is no question but that I am paid more than I would be if my services were subject to true labour market competition.