regr_r2() and corr() outputs differ from the example DBMS

CREATE TABLE t1 (x INT, y int, z CHAR(5));

INSERT INTO t1 VALUES (NULL, NULL, ''),(20, 1, 'aaa'),(39, 2, 'aaa'),(48, 3, 'bbb'),(57, 4, 'bbb'),(66, 5, 'aaa'),(75, 6, 'aaa'),(84, 7, 'bbb');

SELECT REGR_R2(y, x) FROM t1;

0.9814869668246445

SELECT z, REGR_R2(y, x) FROM t1 GROUP BY z ORDER BY z;

aaa	0.9798973217047072

bbb	1

SELECT z, CORR(y, x) FROM t1 GROUP BY z ORDER BY z;

1

SELECT z, CORR(y, x) FROM t1 GROUP BY z ORDER BY z;

0.9999999999999999