Why would a patient's hemoglobin and hematocrit be elevated if he or she has chf?

It isn't: Most of the time, the hemoglobin and hematocrit are not elevated in the patient with congestive heart failure. Rare instances where this might occur include polycythemia vera, excessive transfusion, or a COPD patient with chronic hypoxia who happens to have heart failure as well.

Treatment effect: In point of fact, someone with untreated CHF will have a lower h&h. These measurementa go up with "hemoconcentration" which reflects the volume contracting effects of treatment for chf. Bottom-line: in the absence of an unrelated cause, elevated h&h reflect treatment, not disease.