Thanks for the pictures!!
I think that the panel (at least those in front) have an uneven seam rising from one side to another like this draft. I counted seven panels all around the skirt, three in front and four in the back, so adjust the upper side of the panels according.
Meanwhile i received a message on flickr by Enchantedviolin that bought that skirt and explained
".... you are right, there are no pleats on the top or underskirt - the underskirt is made of triangle and rectangle panels and that is why it gathers and flares out at the bottom. It goes triangle, rectangle, triangle, rectangle etc so that the waist of the underskirt is flat and the same size as the overskirt but much, much wider at the bottom."
this in fact is really helpful, so in a bit of photoshop creativity i produced this draft also for undergown. I think that adding two triangles between rectangles will add up even more fullness at the bottom. Rectangles also are slanted. However I think I should carefully make a muslin playing with bottom circumference, as adding more and more fullness will reach the tutu effect and have too much fabric under the upper skirt (which seems a full circle one)
What do you think?