I think it boils down entirely to pure individual chance. Someone may be a chain smoker and die naturally. That does not obviate the medical fact that smoking increases the chances of lung cancer. It is like rolling a die - you can estimate probabilities but that your dice will land two 6's are purely based on the event. Yet, done over a couple of thousand trials we definitely know it happens 1 per 36 tries on average.

In cases such as your MIL, maybe she's less prone to gastric inflammation. That simple fact reduces her chances of other serious ailments. Its a biological chain reaction after all. Cancer is strongly linked to chronic inflammation.