I’m guessing they aren’t actual Parts of the main Mesh and they are separate.
Check the location of the ‘center’ of the separate meshes in Blender. I bet they aren’t aligned with the ‘center’ of the main Mesh.
By center, there’s a large white dot that usually shows at the 0,0,0 point in Blender. I can’t remember what the term for it is, but when you select eacg separate mesh you will probably see its white dot appear somewhere offset.