Max time queries - Mailing list pgsql-performance

Heres an oddity.  Why would it take more time to not find an answer than it would to find one? 

 

maxTime (Records=0, Time=2223ms)
SQL =
select cr.start_time as max
from call_results cr, timezone tz, lists l
where (cr.start_time between '10/15/2002 08:00' and '10/15/2002 23:00')
and l.full_phone = cr.phonenum
and l.area_code = tz.area_code
and tz.greenwich = '-7'
and cr.project_id = 11
and l.client_id = 8
order by cr.start_time desc
limit 1

 

 

maxTime (Records=1, Time=10ms)
SQL =
select cr.start_time as max
from call_results cr, timezone tz, lists l
where (cr.start_time between '10/15/2002 08:00' and '10/15/2002 23:00')
and l.full_phone = cr.phonenum
and l.area_code = tz.area_code
and tz.greenwich = '-8'
and cr.project_id = 11
and l.client_id = 8
order by cr.start_time desc
limit 1
 NOTICE:  QUERY PLAN:

 

Limit  (cost=0.00..331.03 rows=1 width=49) (actual time=1.19..1.53 rows=1 loops=1)
  ->  Nested Loop  (cost=0.00..1266550.38 rows=3826 width=49) (actual time=1.19..1.52 rows=2 loops=1)
        ->  Nested Loop  (cost=0.00..776978.04 rows=90825 width=42) (actual time=0.84..1.10 rows=2 loops=1)
              ->  Index Scan Backward using start_time_idx on call_results cr  (cost=0.00..6569.39 rows=6693 width=22) (actual time=0.39..0.48 rows=2 loops=1)
              ->  Index Scan using full_phone_idx on lists l  (cost=0.00..114.94 rows=14 width=20) (actual time=0.30..0.30 rows=1 loops=2)
        ->  Index Scan using area_code_idx on timezone tz  (cost=0.00..5.38 rows=1 width=7) (actual time=0.19..0.20 rows=1 loops=2)
Total runtime: 1.74 msec