This comparison is in terms of emissions of nonmethane hydrocarbons and particulates, which is emitted during the process of manufacturing the electric motor, inverters and the nickel metal hydride batteries. In other words, in order to manufacture the Prius, Toyota needs to expand or use more energy to produce that extra electric motor and all of its ancillaries whereas a normal single engined fossil fueled car does not need the extra manufacturing processes.
Toyota had found out during these life cycle emissions assessments that when compared to cars in the same class, the Prius emitted more bad stuff during manufacturing but fared better over its lifetime in use. The Prius is better than the average same sized car in terms of CO2, nitrogen oxide and sulfur oxide over its lifetime. I suppose this meant that Toyota had began its study on the Prius ever since it first started manufacturing it in 1997. A good 13 or so years of information and covers the average 10 year lifespan of any vehicle.
Toyota has stated that it will use the findings to ensure that all of its hybrids, which would include the newly launches Lexus hybrid hot hatch, the CT200h F Sport (photo below) which actually isn't sporty aside from its looks as it only has 134bhp made from the 1.8liter engine and an electric motor.
This may be a dent in the armor for those preaching the virtues of a hybrid. I still feel that hybrid technology isn't really eco-friendly deep down inside. I mean in the age where less is more, a hybrid has TWO engines. Go figure.