Por que MATCH () dá pontuação 0, se houver menos linhas no banco de dados?

Eu segui este exemplo da documentação do Mysql :

CREATE TABLE articles (
       id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
       title VARCHAR(200),
       body TEXT,
       FULLTEXT (title,body)
     );

INSERT INTO articles (title,body) VALUES
     ('MySQL Tutorial','DBMS stands for DataBase ...'),
     ('How To Use MySQL Well','After you went through a ...'),
     ('Optimizing MySQL','In this tutorial we will show ...'),
     ('1001 MySQL Tricks','1. Never run mysqld as root. 2. ...'),
     ('MySQL vs. YourSQL','In the following database comparison ...'),
     ('MySQL Security','When configured properly, MySQL ...');

SELECT *, MATCH ( title, body ) AGAINST ( 'database' ) AS Score
    FROM articles
    WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+------------------------------+------------+
| id | title             | body                         | Score      |
+----+-------------------+------------------------------+------------+
|  5 | MySQL vs. YourSQL | In the following database... | 0.662664...|
|  1 | MySQL Tutorial    | DBMS stands for DataBase ... | 0.655458...|
+----+-------------------+------------------------------+------------+

isso funciona, mas se eu excluir todas as linhas, exceto uma:

DELETE FROM `articles` WHERE id >1

então o select dá Score 0:

SELECT * , MATCH ( title, body ) AGAINST ( 'database' ) AS Score FROM articles