Strange cost computation?

Hi,

Can anybody explain the following results when using EXPLAIN,
one time with enable_seqscan=on and one time with enable_seqscan=off.
What I don't understand is that the nodes created are the same
(index scan, seq scan), but the costs differ.

set enable_seqscan=on;
explain (SELECT alias96.ID FROM JOB AS alias96 WHERE NOT EXISTS
((SELECT alias97.DEPENDENT_ID FROM JOB_MISSING_DEPENDENCIES AS alias97
WHERE alias97.DEPENDENT_ID = alias96.ID)) AND NOT EXISTS ((SELECT
alias98.JOB_ID FROM ACTIVE_JOBS AS alias98 WHERE alias98.JOB_ID =
alias96.ID)) AND ((alias96.STATUS IN (-4, -1))) FOR UPDATE OF alias96
OFFSET 10 LIMIT 10);
NOTICE: QUERY PLAN:

Limit (cost=458.18..916.35 rows=10 width=10)
-> Seq Scan on job alias96 (cost=0.00..2185013.05 rows=47689
width=10)
SubPlan
-> Index Scan using job_mis6970_dependent_id_idx on
job_missing_dependencies alias97 (cost=0.00..893.88 rows=288 width=4)
-> Index Scan using active_jobs_job_id_key on active_jobs
alias98 (cost=0.00..4.48 rows=1 width=4)

set enable_seqscan=off;
explain (SELECT alias96.ID FROM JOB AS alias96 WHERE NOT EXISTS
((SELECT alias97.DEPENDENT_ID FROM JOB_MISSING_DEPENDENCIES AS alias97
WHERE alias97.DEPENDENT_ID = alias96.ID)) AND NOT EXISTS ((SELECT
alias98.JOB_ID FROM ACTIVE_JOBS AS alias98 WHERE alias98.JOB_ID =
alias96.ID)) AND ((alias96.STATUS IN (-4, -1))) FOR UPDATE OF alias96
OFFSET 10 LIMIT 10);
NOTICE: QUERY PLAN:

Limit (cost=100000458.18..100000916.35 rows=10 width=10)
-> Seq Scan on job alias96 (cost=100000000.00..102185013.05
rows=47689 width=10)
SubPlan
-> Index Scan using job_mis6970_dependent_id_idx on
job_missing_dependencies alias97 (cost=0.00..893.88 rows=288 width=4)
-> Index Scan using active_jobs_job_id_key on active_jobs
alias98 (cost=0.00..4.48 rows=1 width=4)

Ives