The train operator’s webpage does not require seat reservations, however eurail shows that it’s required. Why is this? Can I follow the rules from the train operator?

For example, Rome-Florence, 27 january. In the page of trenitalia it is not mandatory to reserve a seat or in case you want to, you have to pay only 2€ while when I want to reserve that trip, eurail, says that is mandatory the seat reservation for that route