Unexpected array_remove results

Hi,
should not in example below array_remove return same results?

test1=# create temp table tmptest (trid text[]);
CREATE TABLE
test1=# insert into tmptest values(null::text[]);
INSERT 0 1

test1=# update tmptest set trid[2:4]='{b,NULL,d}';
UPDATE 1
test1=# select trid,array_remove(trid,
NULL::text),array_lower(array_remove(trid, NULL::text),
1),array_upper(array_remove(trid, NULL::text), 1) from tmptest;
trid | array_remove | array_lower | array_upper
------------------+--------------+-------------+-------------
[2:4]={b,NULL,d} | [2:3]={b,d} | 2 | 3
(1 row)

test1=# update tmptest set trid='{NULL,b,NULL,d}';
UPDATE 1
test1=# select trid,array_remove(trid,
NULL::text),array_lower(array_remove(trid, NULL::text),
1),array_upper(array_remove(trid, NULL::text), 1) from tmptest;
trid | array_remove | array_lower | array_upper
-----------------+--------------+-------------+-------------
{NULL,b,NULL,d} | {b,d} | 1 | 2
(1 row)

I expected that in both results values will start from index 1.

Regards,
Matija Lesar