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

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.
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.