Question on UNION

Everyone,

I ran into something I wasn't expecting while developing a new application. I have two similar tables that are occasionally unioned pulling only about 3 fields from each. During my testing phase I noticed that the union statement was returning what appeared to be a distinct list rather than a pure union such as is illustrated below:

create table t1 (
f1 serial primary key,
f2 text,
f3 text,
f4 integer
);

create table t2 (
f1 serial primary key,
f2 text,
f3 text,
f4 integer
);

insert into t1 (f2, f3, f4) values ('A', 'a', 1);
insert into t1 (f2, f3, f4) values ('A', 'a', 1);
insert into t1 (f2, f3, f4) values ('B', 'b', 2);
insert into t1 (f2, f3, f4) values ('B', 'b', 2);

insert into t2 (f2, f3, f4) values ('C', 'c', 3);
insert into t2 (f2, f3, f4) values ('C', 'c', 3);
insert into t2 (f2, f3, f4) values ('D', 'd', 4);
insert into t2 (f2, f3, f4) values ('D', 'd', 4);

temp=# select * from t1;
f1 | f2 | f3 | f4
----+----+----+----
1 | A | a | 1
2 | A | a | 1
3 | B | b | 2
4 | B | b | 2
(4 rows)

temp=# select * from t2;
f1 | f2 | f3 | f4
----+----+----+----
1 | C | c | 3
2 | C | c | 3
3 | D | d | 4
4 | D | d | 4
(4 rows)

When I leave out f1 I receive a distinct list:

temp=# (select f2, f3, f4 from t1) UNION (select f2, f3, f4 from t2) order by f4;
f2 | f3 | f4
----+----+----
A | a | 1
B | b | 2
C | c | 3
D | d | 4
(4 rows)

When the key field is included I get a full list:

temp=# (select * from t1) UNION (select * from t2) order by f4;
f1 | f2 | f3 | f4
----+----+----+----
1 | A | a | 1
2 | A | a | 1
3 | B | b | 2
4 | B | b | 2
1 | C | c | 3
2 | C | c | 3
3 | D | d | 4
4 | D | d | 4
(8 rows)

Can anyone explain the underlying principle(s) in the UNION that would cause this?

Thanks in advance
Mark