Who can resolve this BUG ?

INM1=# explain select ref_num from grls where ag_id in (24);
Index Scan using grls_ag_id on grls  (cost=0.00..597.87 rows=849 width=12)

IMN1=# explain select ref_num from grls where ag_id in (select 24);
Seq Scan on grls  (cost=0.00..992.44 rows=30195 width=12)
  SubPlan
    ->  Materialize  (cost=0.00..0.00 rows=0 width=0)
          ->  Result  (cost=0.00..0.00 rows=0 width=0)

IMN1=# explain select ref_num from grls where ag_id = (select 24);
Index Scan using grls_ag_id on grls  (cost=0.00..597.87 rows=849 width=12)
  InitPlan
    ->  Result  (cost=0.00..0.00 rows=0 width=0)

". . . in (24)  " work in 35 faster than ". . . in (select 24)". In the
last don't use indices for increase perfomans.
It's  been normaly work in PostgreSQL version 7.2 ? Or not?

P.S.
Sorry my English.