Why creating GIN table index is so slow than inserting data into empty table with the same index?

example:

select version();
version
--------------------------------------------------------------------------------------------
PostgreSQL 8.3.6 on i486-pc-linux-gnu, compiled by GCC gcc-4.3.real (Debian 4.3.3-3) 4.3.3

show maintenance_work_mem ;
maintenance_work_mem
----------------------
128MB

create table a (i1 int, i2 int, i3 int, i4 int, i5 int, i6 int);

insert into a select n, n, n, n, n, n from generate_series(1, 100000) as n;
INSERT 0 100000
Время: 570,110 мс

create index arr_gin on a using gin ( (array[i1, i2, i3, i4, i5, i6]) );
CREATE INDEX
Время: 203068,314 мс

truncate a;
drop index arr_gin ;

create index arr_gin on a using gin ( (array[i1, i2, i3, i4, i5, i6]) );
CREATE INDEX
Время: 3,246 мс

insert into a select n, n, n, n, n, n from generate_series(1, 100000) as n;
INSERT 0 100000
Время: 2405,481 мс

select pg_size_pretty(pg_total_relation_size('a')) as total,
pg_size_pretty(pg_relation_size('a')) as table;
total | table
---------+---------
9792 kB | 5096 kB

203068.314 ms VS 2405.481 ms, is this behaviour normal ?

Thanks !

--
Sergey Burladyan