See https://de.wikipedia.org/w/index.php?title=Wikipedia%3AHauptseite&type=revision&diff=166286800&oldid=165337579&visualdiff=1 (or probably any change to a main page).
Additionally to the changes visible in the wikitext diff, the visual diff also shows changes to the article of the day etc. So it probably uses the date of the old version to generate that version, and thus finds a difference to the current version. IMHO this is inconsistent: If you view the old version, it will use the current date and thus show the same content as for the current version. Visual diff should do the same.