He has a physical condition, hypertension, that's made worse by his psychological stress. There is a physical cause for his disorder. He has a family history of heart disease, he eats a lot of salt and he's overweight. All of these things can cause hypertension. However, his stress is exacerbating his blood pressure. Because Stan's psychological issue is making the disease worse, but is not the only cause of the disease, he does not have a somatoform disorder. But, compare that to his friend Missy.