Seconds precision in timestamp columns

Hi,

In the dvdrental example database, there is a column named
'payment_date' on the 'payment' table and columns 'rental_date' and
'return_date' on the 'rental' table.  These columns appear on the
information_schema.columns view with identical specifications: datatype
= 'timestamp without timezone', datetime_precision = 6. However, a
select on table 'payment' presents column 'payment_date' with 6
fractional digits, while a select on table 'rental' presents columns
'rental_date' and 'return_date' without fractional digits.

select payment_date from payment limit 1;
--> for example, payment_date: '2007-02-15 22:25:46.996577'

select rental_date, return_date from rental limit 1;
--> for example, rental_date: '2005-05-24 22:54:33'

This happens in PgAdmin and also in the query values returned by
PQgetvalue(), with LibPq.
What defines this seconds precision?  Is it possible to find the number
of fractional digits that will be returned in a datetime column, before
calling PQgetvalue()?

Thanks in advance.

Luiz Fernandes