Looks to me as a possible glitch.
I added a Surface Book and Xbox One S with FIFA 17 (500GB) and out of curiosity the Surface Pro 4 and saw that the Xbox One discount was added to both the books.
Microsoft Surface Book - 256GB / Intel Core i5 - £1189 (from £1599)
XBOX - £249.99
Microsoft Surface Pro 4 - 256GB / Intel Core i5 - £721.10 (from £1079.00)