Still, it is not entirely out of my hands. I must still tend to the crop, watering it, weeding it, doing whatever is necessary to its success. If I do this, I can hope for a good crop. But I can't guarantee success. That's up to G-d.
Personal health is akin to that.
A person can get sick if G-d wills it, but a person can also make himself sick, either deliberately or through negligence. A heavenly decree can also decide that you won't catch that bus or plane. If heaven did so decree, it is in fact better for you to have missed that bus or plane. Or it might be better for you in the spiritual sense, in many possible different ways. I won't go into that right now.