The difference in air changes per hour (ac/h) between the sum of mechanical and natural ventilation and infiltration values you entered on in model data and the values reported in simulation results is caused by the method used to calculate and report ac/h in EnergyPlus v4 and later. The new way of reporting ac/h includes consideration of the air density and the exact air change rates reported will depend on the density (temperature) of the outside air being introduced into the building relative to the density of the zone air.