The measurements Rtings produces are always very interesting, but you have to take their conclusions with a pinch of salt. In this case they show a massive spike in THD at 800Hz, going up to 10%. Their raw frequency response shows an anomalous valley at the same frequency. A huge isolated spike like this suggests that something was rattling around and causing a resonance. They should have persisted in trying to get a good fit because their results may have been affected by problems with their measurement setup.