Hello and welcome to the forum. For some strange reason I think you will find that the Venice-Rome 4hr 27mins services that continue on to Naples are known as "Eurostar Italia Alta Velocita" whilst those which terminate in Rome are simply known as Eurostar Italia. The only Eurostar Fast services I could find between Venice and Rome took 3hr 59mins which I believe is attained by cutting out some of the stops.
I think Trenitalia are masters of complication and always find their website an absolute nightmare. If you plan your journey using the SBB website ALL Venice-Rome services are simply referred to as Eurostar Italia
