You need to ***** the color out before you dye it.
I mix my own now to make what I like. It sounds like you should put 1/4 black in with the color you used. The black has blue in it.
You buy the dark auburn take 1/4 out and then buy the black and pour in 1/4 bottle into replace it. I mixed up my own stuff in to 14 bottles and have it ready for the next year.
Cuz then the salons would never make any money.
My guess is because the box can't tell what your ba<x>se color is before you dye it.
I honestly don't think it's BS. It might just be that under store lighting, your hair looks a different color and if you yourself are trying to judge that, it can be really difficult.
Maybe stick to highlights if you are doing a box. Like that, slight differences in the end result won't matter so much.