Is it true that LPG pollutes less than petrol? And if so, why?

First of all it is important to remember that the LPG (Liquefied Petroleum Gas) vehicles in circulation in Italy are almost all modified petrol-fueled cars.
LPG has rightly been considered a fuel with a lower environmental impact compared with petrol on account of lower emissions, particularly of benzene.
In the last 5-10 years however, the development of engine technology has been such that the difference between the emissions of a LPG-driven vehicle and that of the same vehicle using petrol has become practically negligible.