update the same tuple in one command twice

In one command, why can we update the same tuple for twice?
And the result is not predicated!!

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TEST=# CREATE TABLE t1(a INT, b INT);
CREATE TABLE
TEST=# CREATE TABLE t2(c INT, d INT, e INT);
CREATE TABLE
TEST=# INSERT INTO t1 VALUES(1, 1);
INSERT 0 1
TEST=# INSERT INTO t1 VALUES(2, 3);
INSERT 0 1
TEST=# INSERT INTO t2 VALUES(2, 203, 212);
INSERT 0 1
TEST=# INSERT INTO t2 VALUES(2, 324, 1342);
INSERT 0 1
TEST=# INSERT INTO t2 VALUES(3, 342, 214);
INSERT 0 1
TEST=# update t1 set a = t2.d from t2 where a=t2.c;
UPDATE 1
TEST=# select * from t1;
a | b
-----+---
1 | 1
203 | 3
(2 rows)

why not is:
a | b
-----+---
1 | 1
324 | 3
(2 rows)

TEST=# select * from t2;
c | d | e
---+-----+------
2 | 203 | 212
2 | 324 | 1342
3 | 342 | 214
(3 rows)

TEST=#