Music stored on mac is calculated twice
MacBook, macOS High Sierra (10.13.3), null
MacBook, macOS High Sierra (10.13.3), null
iTunes typically creates a copy in the iTunes Media folder when it imports in. So you probably do have two copies of it if you look. One in /Users/username/Music/iTunes/iTunes Media/Music and the other in your documents folder.
The iTunes Media folder is only 345 MB.
The categories are not mutually exclusive. The Documents category, using File Browser as you have, shows everything in your Home folder. As the Music folder is in your Home and contains the iTunes Library, it calculates the size of the iTunes Library there. The iTunes category shows you all of the music files in the Library.
Music stored on mac is calculated twice