In order to assess health and get a fair indication of how healthy someone is, we would have to compare them to other people with similar characteristics e.g age, exercise habits, etc.
Cancer patient in recovery: someone with breast cancer who has had a mastectomy has had their level of health changed relative to others, and to their previous state of health. They may consider their health good during recovery compared to how it previously was, yet still poor when compared to people without cancer.
Changing health over lifespan: Changes to For younger people it may be physical fitness and an active social life, whereas for older people may be completing everyday tasks independently