Почему в первом случае значение 0.6666667, а в двух других 0.5?

=# SELECT word_similarity('abc def', 'abc 123 def') as "similarity 1", word_similarity('abc def', 'abc 123 456 def') as "similarity 2", word_similarity('abc def', 'abc 123 456 789 def') as "similarity 3";
 similarity 1 | similarity 2 | similarity 3 
--------------+--------------+--------------
    0.6666667 |          0.5 |          0.5
(1 row)