Well, IIRC, in those days, when a bishop could not openly function, it was a common disguise to adopt the persona of being just a parish priest (though even that had risks, of course). As part of such a 'cover story" I can imagine - but it is just a guess - that the bishop would have to have a link with some particular parish, presumably the one he claimed to be PP of. And what better one than a mensal parish, of which he was technically pastor anyway?
