Abstract Accurate knowledge of the pattern of interseismic coupling on megathrusts is critical to preparing for future earthquakes and tsunamis. However, published models for a given fault zone commonly differ widely, making hazard assessment difficult. Critically, many kinematic coupling models ignore the effects of realistic boundary conditions and material properties, which may contribute an unknown bias to the model results. Here, we develop a 2D finite element model that incorporates finite slab thickness and variable shear modulus to quantify potential biases in these models. We show that models that do not incorporate a finite slab thickness and realistic material properties, particularly a low‐shear modulus accretionary prism, potentially exhibit a bias toward a shallower locked‐to‐creeping transition at depth. This bias is strongest in areas lacking offshore data and could potentially explain the apparent gap between the inferred depth of kinematic coupling and the location of Episodic Tremor and Slip in Cascadia.