I think your provider record most of the live tv shows and when the show has ended,give you access to this file in the catchup list. It seems difficult to give access during the recording when the file is not complete, if i understand in the right way.
So the catchup is a recording and not a live stream which will be in a different format. That is why I can only record it from when I press record on the Buzzbox and not the whole thing if that makes sense. 🤔 😍