Just a note that "beginner" is game-specific. It means the account hasn't played that particular game, in ranked mode, on BGA before.
No matter how old your BGA account is, how many games/matches you have played before, you always start as "beginner" for every new game you try. Apart from what others have said about people having played the game in real life and therefore not being "true" beginners, there are other ways for someone to excel in their first match:
They may be an expert in some very similar game. For instance, Ticket to Ride and Ticket to Ride Europe are separate games here. Also Through the Ages and Through the Ages (II), or Russian Railroads and Ultimate Railroads, or Boomerang Australia, Boomerang USA and Boomerang Europe.
They may have played many similar (but not as much) games. I'd say mastering many worker-placement games, or many trick-taking games will give you an edge in the next worker-placement or trick-taking game you try.
They may have read some resources about the best strategies in that particular game.
They may have played many matches in friendly mode, solo or with friends.
