The obvious reading is that what follows a q is always a u; the non-obvious and false reading is that what u always follows is a q. I don't know that I can persuade you that both readings are available, but that's what it's about. I dunno if this helps, but I think it's because "follows" in this strict ordering sense implies a biconditional relationship (2 follows 1 and 1 is followed by 2); but using "follow" in that sense is a mixing of metaphors, since r alphabetically follows q and that's the only sense in which the strict-ordering concept of "follow" holds.