Music Bank has released an official statement on LABOUMs win over IU on the episode that aired on April 28.
A source from the program stated that the winner is selected according to the ranking system indicated on the website, and that LABOUM won according to the criteria. They continued, As IU is a very popular singer, and there has been a very positive reaction to her song, we were also surprised with the results of the winner. However, as the winner was selected with data, it is not something that we can change.
The source also explained that the score is calculated based on the previous week when IUs album had not been released yet. As a result, her physical album score was zero, and LABOUM was able to win with their high physical album score.
The Music Bank ranking is determined by combining the scores for digital sales, viewer preference, broadcast points, and physical album sales. On the recent episode, IUs total score was 4165 (digital sales 3816, viewer preference 305, broadcast points 44, physical album sales 0), and LABOUMs was 4546 (digital sales 116, viewer preference 0, broadcast points 2066, physical album sales 2344), giving them the win for the week.