See how a running total differs from a rate of change with daily parcels: 120...

https://speedy-wiki.win/index.php/How_Do_I_Compute_the_Slope_of_a_Secant_Line_Between_Two_Points%3F

See how a running total differs from a rate of change with daily parcels: 120 delivered by Friday is not the same as 24 parcels per day over five days