Yes ,
@scylla explained it very well.
Test involved in MR mostly uncovered Kidney and Lung disease (If any) Other heart condition and Cancer on many body part (If exists) can't be discovered. Until the Applicant himself declare to Dr.
My parents when applied for Super visa , for mother they asked to do Further test for kidney as her creatine level was out of range , so they wanted to make sure she don't required any dalasis soon (although for Super visa it does not matter to Canada govt. as the dependent need to take care of all medical expense ) but not sure why.
IRCC asked a letter from dr. stating as per her condition she don't need dialysis soon we consulted dr. and she took medication and her creatine level came down by few points and dr. did further test and we provided that to IRCC and she passed medical after 3 month of struggle. but this is not always the case.
Some time because of some parameter your parents medical can be failed .( Considering the parents age bracket 70+ there are high chances Medical fail) this is one of major casue for PGP rejection.
Hope this might helped you.
Good luck