Order question

Hey guys, another strange question here..

If I query and order by a field and there are duplicate values in that
field, what makes one return before the other? Just the first one that PG
comes to on the disk is displayed first or is something else looked at to
determine the order?

Example :

hhs=# SELECT ordernum,fieldname,oid from resultstyle WHERE style_id=1001
order by ordernum asc;
 ordernum |   fieldname   |   oid
----------+---------------+---------
        1 | J.jobtitle    | 1197126
        1 | J.inv_id      | 1197125
        2 | J.updatedon   | 1197127
        3 | J.empinitials | 1197128


What makes the record with j.jobtitle appear above te other, when ordernum
is the same?

Thanks!

-Mitch