Those figures are mA/h (miliAmpere per hour) which is independent from voltage. It means how much current the batteries are capable to deliver in one hour.
The nominal capacity is published in mA*h, not mA/h. Also, it doesn't tell how much current the cell can deliver in an hour. The unit itself tells how many hours you can draw one milliampere from it before it's all drained. But the nominal capacity is not actually measured at 1 mA discharge current nor 2300 mA (and that 2300 mAh reading is not accurate at either current), it's measured at something a lot higher, typically 0,2C (5 hour discharge) i.e. at 460 mA for a 2300 mAh NiMH cell. At high currents, perceived capacity is significantly reduced.
This mAh reading is just a rough approximation of energy stored in the battery. Energy is most definitely dependent on voltage, and a current*time value doesn't take it into account at all, which is why mAh values are only good for comparing batteries of the same technology (roughly sharing a voltage discharge curve shape at a given temperature and age) or when using low-tech devices with linear power supplies (unlike our cameras).
The accurate way of comparing battery capacities for use in a specific camera is to test it in that camera using a well defined procedure in controlled climate. Such a test spec exists, and actually a thread on these forums exists as well...
