Differential coefficient of friction (dCoF), defined as the rate of friction increase with normal load, is generally positive due to the real contact area typically grows during loading. Even under conditions of structural superlubricity (SSL) – a state of ultra-low friction and no wear between crystalline surfaces – the dCoF remains nearly zero, always within the order of 10^-3 but commonly positive. In this study, we observed an abnormal yet robust negative dCoF at Au/graphite SSL interfaces. Furthermore, based on direct comparative experiments and molecular dynamics simulations, it is hypothesized that this counterintuitive negative dCoF may originate from the interaction between Au atoms and intrinsic vacancy defects of graphite. This observed negative dCoF was likely a general property of structural superlubricity interfaces with graphite, i.e., those free of third-body molecules. Our observations bring a brand new understanding of the friction behavior of SSL interfaces.