I think if you zoom in (and I would encourage anyone to zoom in) you can see that there’s a couple of things happening. First she’s twisting in such a way as to present both tits and ass to the viewer. Secondly there’s a blurred bit of background, which looks like the shadow of an object unseen on the fence, that obscures the true curve of her back and lastly her front is obscured by the overalls (shakes fist at overalls) so you can’t see her true depth.