What you are seeing there is the difference in episode synopsis between the BBC and other broadcasters. The BBC tends to put the episode number in text at the start of the episode synopsis (e.g. "1/6. Islands:" meaning episode 1 of 6). So the dedup code extracts the name of the episode (in this case "1_6_ Islands") and then the TVDB code prepends it with the series and episode number.
Dave doesn't put episode numbers at the start of the synopsis, which is why you see the difference.
Everything is operating as it should.