Hi Koi, the problem is basically trying to do too much all in one single surfacing operation. If you try to force a sweep to go through too many shape changes it can result in a stressed surface that can have warping in the surface as it sort of tries to simultaneously pivot and stay squished together.

One method that might be good would be to use a one rail sweep like this: (with Twist = Flat option set)



That's going to make a more relaxed surface and then the top can be formed by a separate surface with a fillet or blend to transition between them:




Compare the surface control points between this sweep and your previous one, note how there is a lot heavier points and some strained areas where things are kind of bunching up. When you get the kind of "seam" like you were seeing it's sort of like too many forces applying stretching, shearing and rotation all at once in a small area and making little bumps.





- Michael