My Mother Died Suddenly At 90 Yrs On Wednesday Of

My Mother died suddenly at 90 yrs on Wednesday of last week. I just received her death certificate and the
cause of death stunned me. She was healthy seeing her doctor every 3 months.
Here are the causes of death congestive cardiomyopathy, ischemia cardiomyopathy, and atherosclerotic heart
disease.
My questions are how could this be? Why had her doctor not known? He did bloodwork, she was on high blood pressure medicine, she seemed fine.