If you check the result, which is returned by bdew.HeatBuilding(), you'll find that its sum differs slightly from your input in annual_heat_demand.
To reproduce this, you can add these two lines to heat_demand_example.py here after calling bdew.HeatBuilding() for a single family housing ('efh'):
result_annual_heat_demand = sum(demand["efh"])
print("Sum of heat demand 'efh': ", result_annual_heat_demand)