First of all, Carol of the Bells was sung by West Point Junior Highs Madrigals. When they sung it, they had very good diction. You could hear every word that they said loud and clear, and it sounded amazing a Capella. When you sing a song, it is nice to have good diction. If not, then it would be a bunch of mumbles, which isn't very pleasant.
Next, the Madrigals sang the correct notes. This