Comparison of Strings

I am running
PostgreSQL 7.0.2 on sparc-sun-solaris2.8, compiled by gcc 2.95.2

I have a database table whose contents are

cs302=# select * from author;
isbn | author
--------------+-------------------------
'1565921941' | 'Meyer, Jon'
'1565921941' | 'Downing, Troy'
'0201100886' | 'Aho, Alfred V.'
'0201100886' | 'Sethi, Ravi'
'0201100886' | 'Ullman, Jeffrey D.'
'0878518096' | 'Maloney, Elbert S.'
'0870211641' | 'Maloney, Elbert S.'
'0441865003' | 'Thomson, Amy'
'0312944446' | 'Vinge, Vernor'
'0030860784' | 'Hungerford, Thomas W.'
'0201038099' | 'Knuth, Donald E.'
'1565920015' | 'Gilly, Daniel'
(12 rows)

If I run the following command, I get [the string matches the first two
entries in the table]:

cs302=# select * from author where isbn = '1565921941';
isbn | author
------+--------
(0 rows)

If I change the = to a ~, I get

cs302=# select * from author where isbn ~ '1565921941';
isbn | author
--------------+-----------------
'1565921941' | 'Meyer, Jon'
'1565921941' | 'Downing, Troy'
(2 rows)

Is there something about comparison of strings with the = that I don't
see, that makes it different from string matching with the ~?

---
Michael Schulte Specialist in Computer Science
UM-St. Louis schulte(at)cs(dot)umsl(dot)edu
8001 Natural Bridge Road (314) 516 5239
St. Louis, MO 63121 USA
http://cs.umsl.edu/~schulte