obj.GetType().GetProperties() does not return a PropertyDescriptorCollection, it returns a System.Reflection.PropertyInfo[]. The PropertyInfo class does, as you suggest, represent only actual properties created on the object. A PropertyDescriptor is either a custom concrete child of the PropertyDescriptor class (implemented by the type defining the custom descriptor), or is an instance of the sealed internal class ReflectPropertyDescriptor that uses the PropertyInfo class to provide dynamic invocation of the property.
So for a class that does not define a custom descriptor, you will functionally get the same objects back, though the PropertyDescriptor is abstracting away the PropertyInfo.