Why does "mesh fill" fill more than needed?

Although I heard that ID map isn't strictly necessary, I still baked it with submesh as a color source. Also all meshes were knife cut and separated in blender either way. But when I click on for example an engine part, for some reason it also selects a part of the wing. In another example I click only on the right elevator and it selects almost the entire mesh. What am I doing wrong?