It has to do with the way the publishers are organized. Macmillan is the big US publisher, which is in itself owned by the Holtzbrink group. Macmillan runs, with several levels of directness, several imprints/brands. AFAIK they mainly do this for historical reasons (they bought formerly independent publishers) and to make clear in which market they publish a book (literature, romance, sf, etc.). A better explanation of the structure of publishers has been done by Charles Stross
I tend to tag with any of the publishers, but use separate tags to indicate type of book/genre.