That's the one new thing learned for today.
I read that cheaper 6V batteries use D cells, with the rest being filler space. Same deal with larger rechargable batteries - C and D cell rechargables might not be using all the space available and only have a 'sub-c' cell inside, or even just a AA.
Remember the gun related definition of 'battery'? Same deal, the single electrical device known as a 'battery' can consist of one or more cells hooked up in series, so you add the voltages together.
Zinc(heavy duty), Alkaline: 1.5V per cell
Lithium: 3V
NiCd, NiMH: 1.2V
Lead-acid: 2.1 (Car battery is 6 cells)
LiIon: 3.6, but unstable, requires power management.
For example, your standard 9v uses a stack of 6 battery cells. They're so tiny that the packaging takes up more space than the internals that generate electricity, so a 9V(22.5cm^3,5wh) doesn't actually hold as much power as a similarly sized C cell(27cm^3,12wh). A rechargable 9V will use 7. A lithium one only needs 3 cells, which is part of the reason why it has so much more capacity(11wh).