I knew a guy who was diagnosed with stomach cancer at 27 years old. Never had a health problem in his life, he didn't even know how insurance worked yet. It was late stage and he was given 6 months to live. He was recently engaged before that and they moved their wedding up to 3 months away in order to have it before he passed. But he was gone from us just 6 weeks after the original diagnosis. He fought like hell to survive. He had every reason to. He was diligent with everything the doctors told him and he was gone within weeks despite being given months.
Cancer is horrible and it is unpredictable by its very nature. Cancer is literally at its definition a collection of unpredictable mutated cells. Thats why it is so hard for doctors to estimate or predict. It is unpredictable. Sometimes the unpredictability works in your favor and sometimes it works against you. But cancer cells do not listen to willpower, despite the common narrative. The reality is it is good and bad luck that often determines your fate.
As a secondary anecdote. I have a friend who's mom had skin cancer, a small patch the size of a dime on her hip. Skin cancer is generally incredibly survivable and low risk (in the world of cancers). She had it removed as a simple procedure and thought she was fine. 3 months later she started having periods of confusion or getting lost doing simple things like going to the store for milk, she would end up gone for the hours and hours forgetting why she even left the house and ending up on the other side of town. It turns out it spread to her brain and she died just 10 days after that first episode. Cancer is brutal.