Index not used. WHY?

CREATE TABLE public.rights (
id int4 DEFAULT nextval('"rights_id_seq"'::text) NOT NULL,
id_user int4 NOT NULL,
id_modull int4 NOT NULL,
CONSTRAINT rights_pkey PRIMARY KEY (id)
)

and I created the following indexes:

CREATE INDEX right_id_modull_idx ON rights USING btree (id_modull);
CREATE INDEX right_id_user_idx ON rights USING btree (id_user);

Now the problem:

EXPLAIN SELECT * FROM rights r WHERE r.id_modull =15
returnes:
Seq Scan on rights r (cost=0.00..12.30 rows=42 width=12)
Filter: (id_modull = 15)

EXPLAIN SELECT * FROM rights r WHERE r.id_user =15
returnes:
Index Scan using right_id_user_idx on rights r (cost=0.00..8.35 rows=11 width=12)
Index Cond: (id_user = 15)

Question: Why the right_id_modull_idx is NOT USED at the 1st query and the second query the right_id_user_idx index is used.

I don't understand this.

Thanx in advance.
Andy.