I’m working on a model of a building. I marked seams around the four edges of the roof and in the vertical corners of the building (I’d prefer fewer seams, but for now…). I set the unwrap method to Conformal because I wanted the shapes of the UV polygons to respect the shape of the polygons in the model. When I unwrapped the mesh, I noticed that several of the polygons with 4 vertices in the model are depicted as having 3 vertices in the UV map. The large polygons on the building roof are notable examples. It does this whether I use the angle-based unwrap method or the conformal unwrap method. There are no duplicate vertices.
What am I doing wrong?