A lipo will have a nominal voltage of 3.7v. So, that's why a 2s pack is rated for 7.4v. However, when you charge lipos, they are "peaked" (it's actually not peaked, but good enough term for this topic) to 4.2v per cell, which gives you the 8.4v rating for a 2s pack. That 4.2v won't stay there very long when running though; after a minute or so, the voltage will drop closer to the nominal value. All lipo chargers will charge the cell to 4.2v (unless you mess with the termination voltage setting), so don't worry about getting full capacity.
This is similar to how NiMH's are rated for a nominal 1.2v each, but right off a charger, they are closer to 1.4v per cell.