That's not exclusive to lithium ion batteries, you may just notice it more because you care about those specs in the application that lions are used.
For instance, AA alkaline batteries are rated at 1.5V. The cheap ones are, in reality, about 1.4-1.5V (and much lower mAh). NiCad (or nickel metal) batteries are 1.2V but with a higher kwh (or amp hour, whatever) rating. These batteries all become less useful as their amp hour availability drops, and this is highly dependent on their application.
For instance, NiCad are almost entirely useless in digital cameras as they start out fully charged near the bottom of the voltage requirement of a digital camera; something like a lithium AA will maintain that higher ~2.5-3V range longer due to its higher mAh rating.
I've got a pocket flashlight which takes 14500 cells - basically a rechargable AA-sized lithium. The batteries are 3.6V and last a lot longer than a 1.5V battery in the same light - in part due to the fact that the flashlight has a regulator that drops the voltage to the LED, but also because of the much higher mAh. I believe the lithium batteries drop to about 2.8V before the regulator in the battery 'shuts it off'.
You'll see the same thing in automotive batteries, too: a 12V car battery that reads 10V or less still may be usable for that application if it's got a high amp hour/cold crank rating, and you may still get many hours of auxillary use out of a battery which is already unable to start your car.