The reason is that most hybrid transmissions use two electric motors/generators - one connected to the engine generates electricity to power the car while the other turns that electricity into motion.
This "power split" system works well in the city but it means hybrids cannot run purely on petrol power. The electric generator and motor have to run all the time, even when the petrol engine is doing most of the work.
This is less efficient than a conventional transmission in steady, highway speed driving.
The "dual mode" hybrid transmission being developed by DaimlerChrysler, BMW and GM adds extra gears and clutches to the hybrid transmission to create an all-mechanical path from the petrol engine to the wheels.
In highway driving, dual mode hybrids work the same way as transmissions in conventional cars.
A dual mode hybrid uses the electric motor and generator only for accelerating and recovering energy in braking. At steady high speeds it is driven purely on petrol (or diesel) power.
That might sound like a step backwards but it's actually an improvement in efficiency.
The jointly developed dual mode transmission has four purely mechanical ratios for open road cruising on petrol/diesel as well as the infinitely variable electro-mechanical gearing range of a power-split hybrid for urban driving.