I's definitely due to our postures. I'm no tailor, but I can tell you what's "wrong" musculoskeletally/biomechanically, and it's logical that this would create the fit issues shown.
An orthopod (or an OT or physio) will correct me on the details; I'm relying on my memory of med school for the details below:
Imagine standing sideways and drawing a vertical line down through ears, shoulders, hips, knees and feet (there are specific anatomical reference points within these elements, but I can't remember them). You could actually do this on your side-photo upthread with some simple editing software. In ideal posture, all these elements will fall down the vertical line. In our postures, the pelvis is tilted, pushing hips forward relative to the vertical line, and creating a relatively large curvature in the lower spine, enlarging the hollow in our backs. I bet you often find jackets "kick" in there, even when fitting well from the front, and I also bet that even slim-fit RTW shirts will tend to have excess material at the back while fitting well from the front.
There are other compensatory changes e.g shoulder will be hunched forward and head pushes forward too (think about the effects of that on your jacket fit, and why certain brands will work better than others with that shape, and what alterations you tend to need). The reasons for these musculoskeletal changes are individual, but IIRC the main reason is too much sex. I know that my posture got a bit better after getting into the habit of doing some daily stretching exercises, for example. Not a night & day difference, but enough to be noticeable. But then again, my posture is worse that yours so I notice the improvement more too.
Somehow tailors can work around these problems, both with jackets and trousers, hiding the deviations from view, or at least minimising them. RTW items can't do this, without major alterations (personally, I don't think it's worth it). Even MTM trousers are a major improvement on most RTW in this regard though, which leads me to think it's probably done by playing with the relative front & rear rises somehow. But on that, I'm just guessing.
Interesting information, but I fixed it for you a little bit. So far, this hasn't really bothered me much, as this effect is diminished when moving. Only standing at attention like posing for a picture here shows it to the full extent. I will definitely keep track of how my trousers fit. I do have a slight dip in my lower spine, but my pelvis seems to be pretty leveled. At least that's what I concluded after gazing affectionately at myself in the mirror this morning. We'll see, it's an interesting fit problem to tackle.