Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software "Spark3D."The electromagnetic wave in the simulation was a TE10 mode-2.85 GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY) = 1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.