Because SEGA originally released several pieces of Sonic Generations content exclusively for specific regions or specific retailer pre-orders. Furthermore, a JTAG/RGH console allows you to bypass Xbox Live authentication to install these DLCs without paying (or without reliving the defunct Xbox 360 Store).
(If 5345080B doesn’t exist, run Sonic Generations once to create it.) Generations Sonic -Jtag RGH DLC-