If somebody has appeared on a show that I have seen some of, the show will appear even if I haven’t seen the specific episode the person appears in. This can get quite confusing for long running shows or panel shows which have lots of guest stars, and it’s very uncommon for someone to have watched all episodes from that show.
An example I have experienced, Joe Thomas (Joe Thomas - Trakt) I have only seen in The Inbetweeners, but it says on the website when I hide “Not watched” I have seen him in Taskmaster (I haven’t).
Another issue I have found caused by this is the watched % is incorrect on a “Person”’s page. E.g. if I haven’t watched the entire show it won’t count as watched, even if I have watched the only episode they feature in from said show.