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My short answer is that sweeps aren't very predictable. They have a tendency to wander around if you don't pin them down carefully. They work OK when things are simple and don't change quickly, but they can get squirrely in a big hurry.
If a sweep is on a nice flat surface, it will probably behave very nicely. When it has to follow a 3D path, life gets much more interesting. In those cases it usually helps to have a guide curve that will control the rotation of the profile around the path, but that doesn't always work as well as you would hope.
Since I'm not very successful with sweeps, perhaps you shouldn't listen too much to my ramblings. If I really knew what I was doing I would be giving you detailed advice on how to make them work right.
For the particular shape you were playing with, perhaps a ruled surface is the right choice and a sweep is just an added bit of complication with no advantage.
i'm curious how sweep follow path option orients the profile along a 3d path. as you can see, at the first segment of the path, the resultant surface is normal to cylinder but at the second segment it deviates from the normal surface.
After a little fiddling, here is my description. You know what would be wonderful? If a SolidWorks software engineer could answer this mystifying question. For now, here's my understanding:
Sweeps are a 2D shape (profile) that create a 3rd dimension by following a path. At any point along the path you can define the relationship between the location of any point on the profile, and the curvature of the path. Thus as the curvature changes, the location of the curves on the profile adjusts relative to the curvature changes.
I have attached a model that I think shows how this works pretty well. Notice however, that there is some error, so there is obviously a little more involved. I created a surface sweep using a line as a profile and a 3D spline as a path. Then I selected the path again by using "convert entitites" on the edge of the sweep, and turned on "show curvature". Note that this curvature is always very close to being coplanar with the sweep.
yeah. interesting! there seems to be a relation between the curvature direction and profile orientation especially when there are not many complicated twist and turns.