On history pages for pages on your watchlist some revisions are marked "updated since my last visit". This used to be reset when you viewed the most recent revision of the page (either by visiting the page itself, or viewing the last revision using its oldid, or viewing diff with the last revision).
This has (at least at Wikimedia Commons) changed recently so that visiting old revisions and diffs not including the last revision also resets the updated markings.
I think it was better the way it was before. If you have not viewed the last revision there are still changes you have not seen. That should be marked in the history page and in your watchlist.