Not the mod creator, but I think I can go over a couple things here;
Arrow velocity is modeled ingame but not represented by any user-facing numerical value to refer to. It's entirely possible that the primitive hunting bow's arrows are faster (bigger draw) than the primitive bow's, like a longbow's arrows have been proven to be faster than a shortbow's. Faster arrows means fewer misses on moving targets. Since I don't have access to those values, this is speculation (edit: for BAC's weapons), but it's possibly the reason behind the heavier weight.
Fowling arrows are meant for birds, the lower point damage is enough to kill a bird without mangling it, and I think they're supposed to not go quite as far (according to their description) making it in turn easier to retrieve them. It's also possible that the weight of the arrow has a direct impact on that, and that it was an easy way for the mod creator to control how far they go. Again, speculation on my part.
Boards can get you 20 arrow shafts, so I'm not sure what's going on here. Fletching > Split arrow shaft allows you to select up to 9 shafts to be made at a time, and it removes 1lb worth of wood from a plank for each shaft with the plank getting its durability reduced accordingly, making 20 per plank total.