I think it depends on at least the mining skill of the user, in my case with 70.88 mining skill i get 72.36 Ql shards, this is a 1.48 increase.
== Adding a rare item to an unfinished item as the last item ==
In one case I had a rare sprout and produced a rare fruitpress by adding the sprout as the last item. In another case I had a rare plank and produced a rare shield by adding it as the last item. Pure luck? It's been a good while since I've played last, but since I started again a week ago these 2 rare resources are the only ones I got so far.