Why does Safari sometimes bookmark a different URL?

Then use “Add bookmark” or “Add to Favorites”, iOS instead bookmarks this URL: https://en.wikipedia.org/wiki/UK

The rest of the title is telling it not to redirect as it is a redirect page as is removed.

(Wikipedia then automatically redirects it to https://en.wikipedia.org/wiki/United_Kingdom .)

The page is a redirect page so it redirects it unless you put &redirect=no at the end of the title

“Add to Reading List” seemingly uses this amended URL too, while “Add to Home Screen” uses the unchanged

Add to reading list is the same explanation while add to home screen just copies the link.

I think I found it. It bookmarks URL designated in<link rel="canonical"> tag. Looks like this was done to adapt to Google’s AMP technology, see here.