Both categories involve inquiry, but the location of the obscurity differs.
Khafi: the standard meaning of sariq is evident; its application to tarrar or nabbash must be examined.
Mushkal: the range of idam itself needs interpretation: which foods count?
The khafi question is “Does this special instance fit the known category?”
The mushkal question is “What precisely does the word mean in this usage?”
Keeping them distinct prevents us from treating every interpretive difficulty as mujmal.