Seq. scan when using comparison operators, why? [netaktiv.com #150]

I have an index on column "numero". When, I use a WHERE numero=8,
PostgreSQL uses an index scan (OK) but no when using comparison
operators like numero>8.

essais=# explain select * from pourspip where numero>8;
NOTICE: QUERY PLAN:

Seq Scan on pourspip (cost=0.00..22.50 rows=333 width=28)

EXPLAIN
essais=# explain select * from pourspip where numero=8;
NOTICE: QUERY PLAN:

Index Scan using numero_idx on pourspip (cost=0.00..8.14 rows=10 width=28)

EXPLAIN

Why? MySQL seems able to do it "proprely":

mysql> explain select * from pourspip where numero>8;
+----------+-------+---------------+------------+---------+------+------+------------+
| table | type | possible_keys | key | key_len | ref | rows | Extra |
+----------+-------+---------------+------------+---------+------+------+------------+
| pourspip | range | numero_idx | numero_idx | 5 | NULL | 2 | where used |
+----------+-------+---------------+------------+---------+------+------+------------+