The ink-jet cartridges measure their print out volume based on the number of droplets deposited. A +/- 5% change in ink droplet diameter represents a +/- 15% change in volume. When dealing with really small feature sizes, variable temperatures, and variable viscosities, it is really tough to control droplet diameter exactly. The result is that the ink-cartridge manufacturers need to overfill their cartridges to guarantee that some customer in some corner case doesn't experience a rash of cartridges that run out early.
This tactic is kind of like the hand-soap people that sell a 1 L container of soap with a hand-pump that only works for the first 950 mL. If we can see the soap in the container, we get annoyed because of the 50 mL of waste. However, the ink-cartridge people hide the amount of ink left in the "empty" cartridge, so we don't notice the waste.
Of course, when you are dealing with professional cartridges, and print-outs that can be worth big money, the printer cartridge manufacturers have to guarantee that the ink doesn't run out. The cheapest way to do this is to add a little bit of ink.
In the case of consumer cartridges, HP, Lexmark, and Epson would be deeply upset if a bunch of the customers complained about "empty" cartridges that still said they had 5% capacity left. To prevent complaints, add a little bit of ink ...
Adding a little ink makes everyone happy, until someone actually looks at what is left in the "empty" cartridges, and measures it with sufficiently accurate equipment to realize how much "extra" ink is left.