For 57+ female with Anemia (Hb 11.1) what are the possible causes of low ferritin (39) but decent B12 (498)?

Menstrual blood loss. In women the commonest cause of iron deficiency anemia is excessive blood loss through menstruation. It can generally be made up by taking oral iron supplement and a multivitamin. It would be good idea to see a doctor to exclude other causes.