When you purchase a product and install the ink cartridges for the first time, did you use the supplied / starter ink which came with the product? If you didn't use the starter ink and used new ink cartridges sold separately, it is possible that you feel as though the ink cartridges have run out very quickly. The reason is that the first time you install the ink cartridges the machine will use extra ink to fill the ink delivery tubes, although this is a one-time process that enables the machine to carry out high quality printing. The starter ink has a greater volume than the ink cartridges sold separately for this reason. After this initial setup, replacement ink cartridges will print the specified number of pages quoted.
If you feel that the ink cartridges run out very quickly every time you replace the ink, it's possible you are printing high density documents frequently. The ink consumption varies very much on print density. Especially when you print a document with a higher density of background on it, a bigger amount of ink will be used. Also, the periodic, automatic print head assembly cleaning mechanism will use a small amount of ink each time it cleans itself.