Several methods are known to estimate the own performance compared to other players. Well known in chess is the Elo rating. Scrabble (as well as Scrabble3D) uses the similar NSA rating by the National Scrabble Association. There might be as well regional differences, for instance in Germany.
All methods have in common that the rating will be adopted in respect to the difference between the own rating and the opponent's rating. The probability for a win (or a loss) is multiplied by a factor depending on the absolute rating and the number of games.
With the first registration all players start with a rating of 1000. A game against a player with an equal rating results in plus (or minus) 15 points. If more than one player have taken part in the game the total number is divided by the number of players minus one. That makes for three players 7.5 and for four players 5 points. Every single matching will be calculated and summed up. That makes 3*5 points for the best player, 2*5 minus 5 for the second best, 1*5 minus 2*5 (respectively -5) points for the third player, and minus 3*5 for the player with the least score (the actual value depend on the rating difference). The rating will be stored on the game server and shown in the highscore list.