Another unexpected behaviour

setup:
====
drop table if exists t1;
create table t1 (f1 int);
create unique index uix_t1 on t1(f1) ;
insert into t1(f1) values (1), (2), (3);
select * from t1;

f1
---
1
2
3

test statement:
============
update t1 set f1 = f1 + 1;

In PostgreSQL I got,
ERROR:  duplicate key value violates unique constraint "uix_t1"
DETAIL:  Key (f1)=(2) already exists.

I tried the same test statement in both SQL Server and Oracle
I got correct/expected results as below:

select * from t1;

f1
---
2
3
4

I thought the Atomic should be at statement level, could anyone tell me why
PostgreSQL behaves differently?

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