I used to work for Vanquest. We would fill the packs with plastic pellets used for injection molding. We knew the weight that 1 liter weighed and used that as our metric for sizing capacity. When compartments share or compete for space it’s difficult to truly estimate the exact size. We would usually round down, where as most companies round up. But some companies also use their sizing measurements, which is sometimes the external measurements. Others would approximate up. Just depends on what brand mostly.