Moment of truth – basted the waistband to the top. There is no turning back now.

Facing from the inside. It looks good because I used a piece of selvedge, so it guarantees I sew straight lines with minimal bulk.
And now for the back.
…
o.O
O.o
WTF?!? What on earth is going on here?
I promise it is not hanging sideways. Somehow the waist jumps up on the left, droops wayyy down to the right (noting this is inside out, so on me it would droop on the left and pull up on the right).
Here’s the super weird thing – here’s what happened when I put the dress on:
The pink thread on the left is where I basted, thinking the waist needed to be moved.
But now somehow the waist is sitting more or less straight. If anything, the part that was too low on the form, is now too high on my waist. I don’t get it.
At that point I said screw it and sewed the damn french seam, and figured it was good enough. I’m still confused.
Check the shoulder straps. They might be the source of the hip discontinuity.
It’s also possible that your body isn’t 100% symmetrical. I know at least one person who carries one shoulder higher than the other, for example. Not something you notice everyday, but when you go from form/hanger to body, it can change things.
That is a good point, I’m sure that is some part of it.