According to Digital Spy and some camera savvy TikTokers, the error is noticeable because it's from the HD remastered version of the episode, which has a 16:9 aspect ratio. The show was originally shot in the 4:3 format, meaning the wider aspect ratio is going to show a lot more than was originally intended, i.e. stand in actors who don't quite fit in.

Source: The Irish Daily Star February 11, 2021 01:07 UTC