Yes, it's for different breeds I believe that the only breeds to have to stack with the one foot behind the others (to show off the shoulder's slope to the hips) are the german shepherd dog and that czechovoslakian wolfdog.

Shelties only need to stand straight and tall with their legs kind of apart.

I know it's quite hard to teach unless it's been taught from a pup, I used to try and get Leather and Timber to stack, but it was very hard. I actually got farther with Timber, which was odd, since she has absolutely no patience