Hi,
some illustrations might help to show the difference between spherical and cylindrical mapping:
imagine you're looking from the side into a sphere or cylinder cut open in the middle. Lets assume in both cases an image covering the lower 45° is mapped onto the surface. How is it different?
The first image shows an equirectangular projection ont a sphere. An equal vertical angle will be projected onto an equal distance on the sphere's surface. The points in 3D-space have to accomodate to match. This projection will sqeeze the image towards the top, that's why spherical projections look so distorted when flat.
The second images shows a projection onto a cylinder with a vertical wall. Now an equal amount of vertical pixels will be projected onto an equal distance in 3D. Important point: In the first case, angle was constant, now distance is constant.
Let's see what we get when we put spherical and cylindrical case on top of each other: Case one and two are NOT equal! Mathematically speaking, an angle and the tangent of that angle are NOT equal - they are only simliar for small angles. An image made for one projection does NOT fit onto the other, you will get the distortions you already noticed. And I hope this will answer your initial question: NO, the images I've used for my sceneries are NOT just flat photos.
If you have any doubts whether the output of autostitch, hugin and emblend really produces a spherical pano, please have a look at any of these:
Look at the curvature of straight features above or below the horizon. Could you produce this with anything else than a fisheye-lens or a pano stitching software? Contact the authors and ask them.
Or better yet, read a workshop:
You'll find a lot of articles on Wikipedia, but they're mostly referring to cartography:
Regards