With a little help from Yahoo! and Robert C...

The nominal voltage of a LiPo is 3.7 volts per cell. But, since we want all the performance we can get out of our batteries, we charge them to their maximum safe capacity. Most manufacturers claim this is 4.23 volts per cell. Almost every LiPo charger out there that utilizes CC/CV type charging will end the charge at around 4.21v.

By way of comparison, NiMh batteries typically will charge to around 1.4v per cell, even though they are rated at 1.2v. The battery in your full size car (12 volts) usually will be around 13.5 volts if it is fully charged.

As to why cells are listed at their nominal charge and not their full voltage capacity... I think it may just be because that is the way it's always been done.