Presumably it's because Boston's been, well, dead longer than most. If you look at some of the other Black Lanterns that have been shown so far, it seems the likes of Aquaman, Martian Manhunter, E2 Supes - They all have rotting flesh on their bones, whereas Katma Tui's body has dwindling flesh.
So I think it depends purely on how long they've been dead.