Anomalous ferroelectric-like electronic hysteresis has recently been reported in intrinsically nonpolar graphene/hexagonal boron nitride (hBN) heterostructures and widely exploited to realize electrically switchable superconductivity, correlated insulators, and neuromorphic functionalities. The origin of this anomalous hysteresis is still under debate, hindering its applications. Among the proposals are moiré-driven charge transfer, twist angles between hBN, conductive sliding ferroelectricity and dielectric defects. Here we study the detailed hysteretic transport properties in dual-gated hBN/graphene/hBN heterostructures. Two devices that share the same moiré and twist angle configurations exhibit strikingly distinct hysteretic responses, allowing us to unambiguously rule out moiré-induced and twist-angle-related mechanisms. Moreover, the ratchet-like charge injections give rise to unconventional hysteresis that is pronouncedly asymmetric and continuously scalable with the sweeping range of external electric fields, in sharp contrast to conventional ferroelectric behavior with fixed coercive fields that is symmetric with respect to zero field. These observations challenge the theoretical scenario in terms of spontaneous interlayer electrical polarizations and lead us to consider the defects within hBN flakes. Our analysis and simulations suggest that the interplay between the defects and conducting channel plays an important role in inducing the ratchet-like charge injections and anomalous hysteresis. Our work provides more insight in the origin of electronic hysteresis in graphene-based van der Waals heterostructures, and the observations of asymmetric and scalable electronic hysteresis could lead to new-generation ultrafast memory and synaptic devices.