Re cell voltage, most non-rechargeable designs have a nominal voltage of 1.5V per cell, whereas most rechargeable designs (specifically NiCd and NiMH) have a nominal voltage of 1.2V per cell. You get higher voltages by putting cells in series, so a 9V battery is a series of six 1.5V cells. Since 9 is an exact multiple of 1.5, I can't imagine why battery manufacturers would choose to use only 5 cells and have it at 7.5V, especially since PP3 batteries are always specified as 9V.
For rechargeables though it ain't so simple. Some might use the same six-cell design but with rechargeable cells, but that only gets 7.2V. 8.4V needs seven cells so needs a different design, which presumably would need different tooling.