a problem with data sorting

Hi,

I 've got a data sorting problem when running PgAdmin II v1.2. Providing
there is simple table, called "test_xx", which has only one column, called
"aa" CHAR(16), and the column is set as primary key. Put in some data such
as following.
( _ stands for a space character)

'AA_B'
'AAB'
'BB_123'
'BB123'

After execute ...

SELECT * FROM test_xx ORDER BY aa;

I 've got ...

'AAB'
'AA_B'
'BB123'
'BB_123'

How can 'AAB' < 'AA_B' and  'BB123' <'BB_123'?

I run it under windows xp. Does it matter?  Thnak you!!

Jack