aggregate on zero rows slow?

I am trying to figure out why given the same query:

SELECT MAX(fromoctets),MAX(tooctets)
  FROM stats,trans
 WHERE stats.id=stats_id
   AND firsttimei=9224192
   AND sourcepeeraddress='xxx.yyy.zzz.38'
   AND flowindex=4701
   AND timeslice<'15:20:00 Wed  1 May 2002';

Aggregate  (cost=57.36..57.36 rows=1 width=16)
  ->  Nested Loop  (cost=0.00..57.36 rows=1 width=16)
        ->  Index Scan using firsttimei_idx on trans  (cost=0.00..54.33 rows=1 width=12)
        ->  Index Scan using stats_pkey on stats  (cost=0.00..3.02 rows=1 width=4)


the select appears to take much longer to complete if the WHERE clause
matches zero rows, than if it matches some rows.

Is this possible, or just chance?

Cheers,

Patrick