Indexes works only on miss

Hi,

I really don't understand this behaviour. I have a table with column "owner"
on which I created an index with btree method. The table contains around 3k
rows.

Now I run it using EXPLAIN command.

This query has some results:

explain SELECT "Name" FROM test WHERE "Owner"='root'

"Seq Scan on test (cost=0.00..119.11 rows=263 width=11)"
" Filter: (("Owner")::text = 'root'::text)"

Query without results:

explain SELECT "Name" FROM test WHERE "Owner"='blah'

"Index Scan using idx_test_owner on test (cost=0.00..96.56 rows=28
width=11)"
" Index Cond: (("Owner")::text = 'blah'::text)"

Why is this happening? Is it because of the memory? I'm running on default
db settings, version 8.0 and SUSE 10.

Thanks for help, Sebastjan