Have you red the tutorials or looked at wikipedia or howstuffworks.com, or the Toyota website? They all explain pretty clearly why Toyota hybrids get better mpg in local driving than the highway.

Similar systems are used on the Camry, Highlander, Escape, and Prius. They are able to use only the electric motor for propulsion at lower speeds and moderate acceleration- the gas engine is off. That saves more fuel under those conditions, which are the conditions seen in 'city' driving. The highway is faster and the engine is on almost the entire time.

This is different from the Honda system which only uses the electric motor to assist the engine- the engine still has to provide propulsion all the time.