... and even harder to describe without images.3 datum planes, X_DIR, Y_DIR, Z_DIR. I make an extruded surface sketched on the X_DIR plane both sides. The profile is like a tilde ~ , or a flattened S shape lying on its side. IOW it goes from convex to concave. I will call this surface "S".
In the Z_DIR plane I do two sketched datum curves, similar but not quite symmetrical. They were but now they're not, thank you designer. They are projected onto the surface "S". From the sketching plane of these 2 curves surface "S" looks like a rectangle.
So we have 2 similar-looking curves projected. On each one I make a variable section sweep surface, normal to projection, picking the Z_DIR plane (the one I sketched the curves on) as the direction reference. The sketch for each surface looks like an arrow head with the aspect ratio the wrong way around. So from the center of the sketch there's one short line going up at 3 degrees to the vertical, and another going down at 3 degrees to the vertical. IOW, one segment like " / " when the center is at the bottom, and another segment " \ ", when the center of the sketch is at the top. LIke a chevron on its side. The apex of the chevron runs along projected curve. The lengths of the two segments are the same.
If I offset this surface, shouldn't I expect the edge of the chevron on the offset surface to lie on the same surface "S", as the original variable section sweep? It's out by .003" in some places.