Why is it so ?

Your non-rechargeable AAA chemistries (Carbon-acid) have a single cell voltage of 1.5 volts. There are six of these in a 9V battery and four in a lantern battery.

Rechargeable chemistries (Nickel Cadmium, Nickel Metal Hydride) have a single cell voltage of 1.2 volts.

Other chemistries can be:
- Lithium 3.1 volts
- Lead acid 2.1 volts (there are six in a car battery)
- Lithium ion (and lithium polymer) 3.7 volts
- Citric acid/zinc (a lemon and a galvanised nail): 0.9V
- Citric acid/magnesium (lemon and magnesium): 1.3V

It basically depends on the chemicals that store the energy. usually the further the electrodes are away from each other on the galvanic table the higher the nominal voltage.

Further to the good examples given above, this is the basic answer to your question:
Different cell chemistries mean different metals in the makeup of the electrodes. Different metals have different affinities for the electrons which drift between the electrodes. Different affinities mean different electromotive forces (EMF) acting on the electrons. EMF also stands for potential difference, or cell voltage, which therefore also varies with the metal composition of the electrodes.

BTW, the discharge rate of rechargeable batteries is generally greater than that of non rechargeables.
I find it more convenient to buy new non rechargeables every now and then, than having to recharge the rechargeables on a fortnightly or monthly basis.

cheers, Peter

Its a little more confusing than that - you can also buy re-chargeable AAA 1.5v batteries , this is what we use to preserve the output power of devices such as hheld cb radios designed for 1.5v cells.

These cells can typically only be recharged about 25 times , but mine get lost by then usually.

Most NiMh and NiCad rechargable cells measure 1.4V when fully charged. Most alkalines read 1.6V. The discharge curve for rechargables is very flat - they will hold their voltage at 1.2V for a long time, while delivering high currents, then it will suddenly drop to zero so you get little warning that they are flat. Alkalines have a more linear relationship between voltage and capacity, and can't deliver high currents.

So they are different batteries for different applications.

I'd suggest that you'd only want to use alkalines in remotes because they have a low spontaneous discharge rate and the power consumption is very low.