From what I know, the 1.2V of NiMH has something to do with voltage fluctuations. The nominal voltage is indeed 1.5V, same as Lithium, Alkaline and drycell but the voltage fluctuates anything from 1.1V to 1.4V when it is in use, so average gives you 1.2V+ and thus 1.2V rating.
The voltage fluctuation is also the reason why NiMH is not suitable for use in some devices that are more sensitive to voltage changes, in which you end up getting very poor battery life from the NiMH, when in actual fact, there is still a lot of juice left when you use in other devices.