Weird. The page numbers listed there for the "basic rules" are usually wrong... It seems the sources take a certain order of precedence, so if something appears in the basic rules, that's what's listed there - but if it also appears in the PHB/DMG/MM, then the backend system seems to include that as a source as well.
If there's no page number for the basic rules entry (which I think there never is, since it was before DDB started actively citing page numbers for sources), but there is a page number for another source it appears in, then DDB seems to show "Basic Rules" but displays the page …