Recently, the Kapitza-Dirac effect has been explored using a pump-probe scheme with ultrafast standing light waves, revealing delay-dependent diffraction fringes in the electron momentum distribution [Science , 1467 (2024)]. These fringes have been interpreted as quantum interference patterns arising from stimulated Compton scattering. However, we find that an ensemble of classical trajectories initialized by sampling the electron’s initial Wigner distribution can accurately reproduce them, with constructive interference corresponding to the convergence of trajectories in momentum space. This quantum–classical correspondence can be understood in terms of the specific geometry of the evolving quasi-one-dimensional phase-space distribution. As the interaction time increases, increasingly nonlinear electron dynamics deform the phase-space distribution into a complex winding structure, and the classical ensemble captures the subsequent evolution of the diffraction patterns only qualitatively. These findings provide a phase-space interpretation of the origin and temporal evolution of ultrafast KD diffraction fringes and may inform coherent electron beam control and electron microscopy.