Actually, I read somewhere that when medicals expire after 12 months, and the processing is still less than 15 months, the officer will send a request to their internal doctor to get an extension approved, stating extension period they want (3 months, 6 months etc.). If the internal doctor is satisfied with the previous medicals, they may simply issue an extension. However, they generally ask for some extra medical tests to be done (e.g. chest x-ray), and they send a letter directly to the applicant requesting this. This email comes from Delhi Medical assessment department, not from immigration department. Once the internal doctor receives new medicals from panel physicial, they approve extension and visa officer receives internal notification to proceed further. They then issue PPR to applicant generally.
If processing is more than 15 months, the internal doctor/immigration will ask for full remedical, not partial.