Oof. That's mentioned as one of the fucky things about crafting in general. Notice how in vanilla adventure boecarving, everything you can make would logically only require a single bone. If you tried to make a recipe demand more than one bone, bar of metal, piece of cloth etc, it'll work fine in fortress mode but start acting up in adventure mode. Usual result tends to be it'll demand a separate stack for every unit requested, then take 1 from each stack, which would get downright fucky once you get to armor and other things that logically should be taking several bones at once.
And then you get to leather goods, where every hide is just one single unit and therefore 1 corpse will always equal 1 leather item, and at that point it ends up being a choice between:
1. Letting the player make hundreds of dragon bone goods from a single dragon yet only ever getting 1 scale item from each.
2. Needing several stacks of filler to actually use those dragon bones for armor (and still being able to make a ton if you do juggle this requirement), with high risk that it's going to pick the weaker bones as the primary material (meaning none of the benefits).
3. Just exploiting REACTION_CLASS so that at least it's consistent relative to how leather is handled, while also sidestepping all of these conundrums.
It's even worse with bars (average product size 150), cloth (average product size 10000), or thread (15000) for rather obvious reasons, and that's why in Adventurecraft the reactions are exploitable as all hell if you use cloth/thread/bars/sheets/etc spawn in sites instead or handmade ones (the reactions produce products with a unit size of 1 and consume only 1 unit to avoid these shenanigans).