National Lampoon's Winter Holiday

Why is this title shown as the headline title, as opposed to showing the original title? The guidance seems to suggest it should be called National Lampoon's Christmas Vacation. more than 90% of the world won't know it by this name either, as only the UK and Ireland got it with that title. All DVD and Bluray releases in the UK have been with the Christmas Vacation title. I even saw it in the cinema, and don't recall it having that name.