Parmandur
Book-Friend, he/him
The main place I've seen that terminology is in Magic: the Gathering design, where Mark Rosewater uses it to distinguish between card Sets that were built "bottom-up" around a game mechanic concept that narrative tropes were built around (classically, the Ravnica world was built around dual-color interactions in the game space), as opposed to card sets that were designed "top-down" around a narrative theme where card mechanics are selected to re-inforce the tropes: Innistrad, the Gothic Horror set, is the classic example of the latter.Perhaps I do not understand the terms used .... Top down sounds like it means not having a consistency of structure meandering all over the place because you didnt build the bones first.
Top-down doesn't mean sloppy, it just means that mechanical decisions follow from the fluff. And bottom-up doesn't mean that the narrative elements must be lacking, just that the fluff is an add-on to the game mechanics. Either approach can be useful, depending on context.