Why won't the "arrange tabs by" feature work?

I keep selecting to arrange the tabs by Website, not Name, yet the list of open windows won't budge, they are arranged by name of the page, and not the site. I don't always remember the exact title of a page, but do know the website, yet not being in that order, makes searching for the page I want quite time-consuming & frustrating. Why do some of the more basic features of Safari, the Cloud, or the OS work as they're supposed to? I don't get it.