I am saving a couple of 3d variables monthly and their values at a chosen specific depth daily because writing out the entire 3d variable daily is too expensive. I am attaching a plot below that shows the difference (in %) between: (i) the daily value at the chosen depth averaged over a month (from the daily file); and (ii) the monthly value at the same depth (from the monthly file). The top panel shows this difference for the variables

Shouldn't the difference in the plots below be close to zero? I am finding them to be within 0.01% of each other for

**WTT**and**KPP_SRC_TEMP**. For**WTT,**I am writing the daily value at the shallowest model level and for**KPP_SRC_TEMP**, the corresponding depth is 40 m. The bottom panel is zooming into the Kuroshio region.Shouldn't the difference in the plots below be close to zero? I am finding them to be within 0.01% of each other for

**WTT**and 0.001% for**KPP_SRC_TEMP**. This might not be too large but why is there any difference at all? To be consistent, I would have thought averaging the daily values over a month should be equal to the monthly value to within machine precision.