You're missing that they don't really care that everything makes logical sense. In fact, it's an advantage if it doesn't. It forces people that really dig into things to come to a logical contradiction on small things and trains them to get used to discarding their own reason and yielding to the cult.
I think the official doctrine is something like this: Jesus appointed 3 slaves to look over his crap, the ones that did so in a wise manner were declared to have been "faithful and discrete" and at that point they are appointed over "all his belongings." Right now the GB is merely appointed to look after things and assuming they do a good job they'll be declared the FDS. Since they've been using the term FDS for so long, though, they find it convenient to continue this usage as an assertion of their authority.