As far as I understand, you can not sponsor just one parent, the way parents sponsorship works is if your parents are still married to each other, then you can NOT sponsor just one of them. That might be the reason that IRCC sent the medical request for both of them.
Also, once the medical result is passed, you might get a notification from IRCC to pay fee for your mother also. Perhaps, this is very strange that IRCC officer didn't notice in your file that you are sponsoring just your father, I would say that luck was on your side that they didn't reject the file. My suggestion is to get medical for your father and mother done.