Mint, if you can tell a difference in flight from 15 grains you are a much better shot than I ever was/will be after 60 years at this game. More power to you.
Many times, when I practice will have arrows of 50-75 grains or more differences. If I do my part, they all go to the same place. Then again, the distances I shoot at are under 20yards and I would need a scale to tell you which is heavier than the other, even tho the arrows don't look the same. And that will include mostly wood but sometimes carbon and a couple sizes of aluminum.
Same thing goes with spine-all these arrows I shoot are not the same 5# spine group-they may be 10-20# differences in spine. But I will say they are all stiffer than my bow weight at least.