Confusing mass balance about rainfall

Subsequent to the example about rainfall (Why "no rain contribution" even using "1 m/d" rainfall? – EE Modeling System), I calculate the mass balance (Fig-a). The whole time is 3 days. Rainfall is 1 m/day, no evaporation. Inflow = outflow. The domain is 1000m500m. Intial water level is 1m.Water level in Day3 is 4m, which is right. When I choose the mass balance tool, “Start” is 1000m500m1m=0.5 Mil m3, “End” is 1000m500m4m=2 Mil m3, “Delta” is 2-0.5=1.5 Mil m3. These are all right.However, when it comes to rainfall, I cannot understand the number “0.0104”. Because the rainfall is 1000m500m*(1m/day)*(3day)=1.5 Mil m3, not 0.0104?