That is probablay a modeling choice made by the creator, to my knowledge there is no obligation to have a piece of clothing precisely following the contour of the body at zero position. It may be that when posing the mesh deforms somewhat but that is caused by the influence of the bone's blend zones across the different body parts. I have made puffy sleeves that in no way followed the natural shoulder shape, and Poser handled those quite well.

Dominique Digital Cats Media