Getting reassessed, don't understand why

Sorry to bombard you looking for advice but I am concerned and feel you are the best people to advise. I am receiving pip care component due to chronic pain from a back injury. I was then diagnosed with heart failure and sleep apnoea which I made dwp aware of. Once all tests were complete and I had a full diagnosis I requested my pip claim was looked at again and completed a new form. At the assessment the lady looked at my previous problem and did nothing too assess both of the new ailments. I was refused a change failed a mandatory reconsideration and applied for appeal. Whilst they were obtaining medical records from my gp I found that the conditions were infact well under control with treatment and not wanting to face a tribunal I cancelled it. Now today Atos have phoned to say they have to come to my home and do another assessment as requested by dwp. This I do not understand and neither party will tell me the reason for this. Many thanks in advance for any help