Why Brown on Black, Shoo la rue?

I bought this denim dress on sale at Shop La Rue, a moderately popular midrange fashion shop, a few months ago and have fantasized about changing out the buttons ever since.

The button on the left, brown and sewn with a brown thread on a gray/black denim dress, is what it came with. Twelve of them to be exact, including the ones in the pockets and sleeves.

Why? What color story is that? I might be picky but it feels like black and brown only get to hang out on the same garment if there's a print or pattern that brings them together, usually that involves at least some other neutrals. Black and gray should move towards a cooler neutral though, while brown should include a warmer beige/tan tone.

I'm sure these were just cheap and available. I've replaced them with the silvery buttons on the right, bought at Sanki for 220yen. Huge improvement for a small price.