The old fashioned German Shepherds were built like that - before all the over breeding and tinkering that gave us the sloping rear quarters and joint problems! This one looks a lot like Purdue, my first Shepherd, and bears a lot of resemblance to Bimmer, who as most of the old hands around here know, is a GSD/wolf mix.
I think you just lucked up and got a good Shepherd from the old fashioned bloodlines.