My ferritin was 3, 000 at diagnosis and was dropping steadily approximately 30 points per treatment, phlebotomy, but suddenly dropped 500 points. My doctor and I don't understand how this could have happened?

Ferritin. Is not perfectly correlated with iron burden. It also changes with infection and inflammation. Any questionable value should be repeated 6 weeks later.
Recheck. I would recheck. It is possible that your body is now becoming more fully depleted of iron. Also, ferritin is an acute phase reactant so it can be elevated by other things (infections, cancers, etc). However, in this setting i would recheck to see if accurate and, if so, be happy that your frequent phlebotomy may soon be coming to an end.