In reply to timjones:
More seriously, I think it's because in a UK-wide award, it would seem odd to be passed if you were lacking experience of the areas where the vast majority of 'winter' happens. I think they also say something about the relative remoteness and seriousness of Scottish mountains, though obviously you don't necessarily have to go remote in Scotland.
I kind of agree with you, that there's no reason you can't have experienced a pretty full range of UK winter conditions in Wales or the Lakes. But if you haven't done much in Scotland, how do you know that?
Post edited at 10:42