All the ITVs are also carried on Sky, but you just don't get to see them in the menus. (You can select them via the 'other channels' setup, but it's a lot of fiddling) There is, in many areas, a 'secondary' ITV, that you can see on the menus, it's the next nearest region. We are in the ITV Central (or whatever it's called now) but we also get to see ITV Yorkshire on the menus, as a backup.
Your card identifies where you are, and the box is set so as to select your regional ITV when you choose 103. It's just this selection process that Sky can change.
BBC works the same (although they generously allow tyou to select any), your local one is associated with 101.