UPDATE execution time is increasing

Hello!

I would like to ask following question:
I have created a table and I updated all records.
And I executed this update command again and again....
Execution time was growing after each step.
I cannot understand this behavior.
First update command took 6 sec, 30th update (same) command took 36
sec (6x times greater value!!!).
Can somebody explain me why increasing this update time?

-- 1st update: 6175 ms
-- 5th update: 9265 ms
-- 10th update: 15669 ms
-- 20th update: 26940 ms
-- 20th update: 36198 ms

PGSQL version: 9.1.5, parameters: default install used

Thanks your answer in advance!

SCRIPT:

DROP SCHEMA IF EXISTS tempdb CASCADE;
CREATE SCHEMA tempdb;
SET search_path TO tempdb;

DROP TABLE IF EXISTS t;
CREATE TABLE t (
id SERIAL ,
num int NOT NULL,
PRIMARY KEY (id)
);

insert into t
SELECT *,0 FROM generate_series(1,100000);

update t set num=num+1; -- 1st update: 6175 ms
update t set num=num+1;
update t set num=num+1;
update t set num=num+1;
update t set num=num+1;

update t set num=num+1; -- 5th update: 9265 ms
.....

update t set num=num+1; -- 10th update: 15669 ms
.....

update t set num=num+1; -- 20th update: 26940 ms
.....

update t set num=num+1; -- 30th update: 36198 ms
.....

----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.