Thanks for the explanation, Michael. I was never sure what it really tells me and also thought it at least indicates that sth. should be done differently.

Although I have to admit that the created faces' deviating shapes mostly look a bit better than my curves and that it is hard to judge whether a curve looks okay when you have a filling that looks a little different. But I haven't looked at any models in Blender yet. I guess that is what I should do next.