As someone who has only bred dogs in the past, will breed my first chickens next year using the better quality birds I get and raise this year, I would offer this:
To the best of my knowledge, in dogs and presumably in chickens, there has never been a perfect one. "Breeding
to the Standard" means you are always reaching toward that perfect bird using, absolutely, those which most closely represent it from your breeding pens - but - since none is perfect, you don't necessarily just take the two closest and mate them, you evaluate how they fall short of perfection and select mates which have the potential to offset those shortcomings.
In double mating, as I understand it anyway, with certain breeds as previously mentioned such as Barred Rocks, when you take two well-marked birds in a single mating, you may find yourself with smutty-marked offspring rather than nice clean barring. One is therefore using mates that will produce chicks closer
to the Standard.
We are not working from, but to (toward), the Standard.
My two cents. I shall now return to lurk-and-read status