When you purchase a new printer, there is no ink is present in the ink tubes, only the quality control fluid, therefore the printer will need to perform an initial ink charge. Upon inserting your first set of cartridges, the printer will use the ink to fill the ink tubes, print head tank and print head. This process roughly deducts 20-30% of a standard sized cartridge. This would be less for the black cartridge or when using high capacity cartridges inserted in an SG800.
This ink however is not lost and remains in the printer. Once your initial set of cartridges has depleted, the next set you insert will show a level of 100%.