Heart rate monitors sometimes double-count beats, counting the atrial spike as a separate beat from the ventricular spike. As an RN who worked in cardiac care, we had a motto: "treat the patient, not the monitor." When you see an anomalously high (or low) reading, directly palpate a pulse to verify.
A true heart rate of over 200 would likely be symptomatic, as cardiac output will decrease if the rate gets so high the ventricle can't fill between beats.