O.T. Mason, who personally studied a vast number of Native American archery items had this to say in his North American Bows, Arrows and Quivers (1893): "”The arrows of one quiver agree, absolutely.”
Others who have studied a large number of Plains arrows at the Smithsonian etc. report that the arrow shafts are strikingly uniform in diameter, both within a shaft and across a large number of them. Arrow shafts were sized to particular, proven sizes, and when made from particular, proven raw materials, ended up with similar characteristics. Among many NA peoples, arrows were made by dedicated specialists, precisely because making good arrows was so important, and so difficult.
Yes, they had a different mindset and priorities. But even with no PETA in sight, you need to kill the game animals efficiently and predictably to feed your family. An arrow that flies erratically, whether due to having a wrong spine or other reasons, does not penetrate nearly as well, or hit nearly as precisely, as a good arrow does. You don't need to shoot the biggest land animal in North America like the Plains peoples did, to learn this real quick.
Not all museum quivers contain matched arrows. Absent detailed collecting histories, one can always suspect many are not original in composition but rather indiscriminately put together. Also true that different situations called for different arrows. A long-range fighting arrow was thinner and lighter, and with a smaller head and fletching than a buffalo arrow, for instance.
Many of the big game iron arrowheads from the Plains were seriously heavy. The various repros I have weigh from 190 to 250 grains a piece. Coupled with a short, thin hardwood shoot shaft, these arrows end up weighing around 500 grains or more, with a massive FOC %. Stone arrowheads are much lighter, though, better fitted with longer, heavier shafts to make up for the difference. Even with the 48 - 50-inch selfbows I make and shoot, under 10 grains per pound simply lacks the oomph.
Tuukka