One thing to point out about 7994: the lace overlay is not actually darted, that is all in the lining for the sample which is causing some of the discussion. It's designed this way. I would also wager this lace has a high lycra content, so it looks "smoother" when worn. This also shows why foundation garments are useful: some of the bunching on the waist and back would smooth out as well.
As to the model's back, I imagine it is a cross of several things one of which is the dress is a bit too large overall for her. She is standing with her shoulders pushed back and weirdly to make it look like it fit (these dresses aren't really tailored for a model, they make a size and the model fits in it basically) and that in turn causes all the lines to form which in turn causes it to look small. She may also be broad shouldered or broad backed and adding additional problems (arm scythe bunching) and the fabric used in the dress seems to be a pongee or single knit lining, which in RTW causes many of these exact issues.
Not saying there is not issues with this dress as they present it, because there certainly are (the arm scythe is always one of my huge complaints with commercial patterns and one I usually redraft to correct is the big one).