The sampling accuracy used in Tanner Waveform viewer is based on the internal time-step used for running the transient simulation. T-Spice uses variable (dynamic) time steps during transient simulations. Therefore, the time step will be small when signals are changing rapidly and the time step will be large when signals are changing slowly. This allows the simulation run faster. You can adjust the maximum time step allowed for your transient analysis to force more accuracy in the Waveform Viewer.