When you purchase a new printer, 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 set of initial starter cartridges, the printer will use the ink to fill the ink tubes, print head tank and print head. As the set of cartridges that initially comes with your printer are only starter cartridges, this means that your ink levels will be a lot lower than expected and you will therefore not be able to print as many pages as you might expect prior to having to replace them.
The ink used during the initial ink charge is however 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%.