The moiré potential induced by nonuniform interlayer coupling in a twisted van der Waals bilayer manifests itself in excitons, trions, and many other exotic electronic and optical properties, yet its origin remains elusive. Strains are generally believed to give rise to the moiré potential in a complicated way through lattice deformation. Our density functional theory calculations on twisted WSe₂/MoSe₂ heterobilayers () confirm that lattice deformation plays a dominant role but reveal that the marginal normal strain makes only minor contributions, and that the shear strain is not correlated directly to the moiré potential. It is found that the vorticity of atomic displacements, that is, the local lattice rotation, rather than strains in the moiré cells, determines, to a large extent, the moiré potential. This discovery sheds new light on the understanding of the roles of distortion in twistronics and will be instructive in electronic structure engineering of moiré materials.