It’s simple : Bellissima was the in-house codename for the Centenario before its official announcement. (Centenario isn’t in the car list for this reason).
This is the same for the 2017 Ford GT who was called “Phoenix” before its announcement, and is then called by this name in the binaries of the game.

Alright, this deserves a bit of clarity. The Centenario doesn’t show up, the Bellissima does. While the 2017 caught me off-guard, I’m pretty sure the Bellissima is the Centenario because the car hadn’t been revealed while the game was in development. This kind of thing is far from uncommon. I wouldn’t have arrived to…

Knowing is half the battle. -Former Volkswagen owner, then Honda Civic owner