Spot the difference!

From a distance, a black sock looks like a sock, but how is it that they're all slightly different?? In the past, Liam has bought sets of 5 black sock, all with a different colour heel, so you have to match up the exact sock with the exact sock. I've since asked him to buy sets that are plain so that any of the 10 socks can be matched, but eventually, they still seem to be slightly different. How annoying! Bill sported such an example tonight - from a distance they look black, but closer inspection shows an irritating difference in the distance between the thin white line??? Do sock manufacturers do this to send us insane??