I think it's because Edinburgh are more confident in the way they deal with applications than the way in which A-levels examine students. They think they can tell whether or not a student is good enough, and so no need to really press them to get high grades to secure their place.
I suppose it could also be something of a ploy. By offering lower offers than institutions of a similar calibre, they're more likely to be chosen as an insurance by students who would do well at the university? That's pure conjecture, though.