update with row_number() giving unexpected results

Hello,

I'm getting unexpected results doing the following update statement :

create table test(a varchar2(10));

insert into test values('ab');

insert into test values('a_b');

insert into test values('_ab');

insert into test values('!!ab');

insert into test values('!!abc');

insert into test values('bc');

insert into test values('abc');

create table results (a varchar2(10), rn int);

insert into results(rn) values(1);

insert into results(rn) values(2);

insert into results(rn) values(3);

insert into results(rn) values(4);

insert into results(rn) values(5);

insert into results(rn) values(6);

insert into results(rn) values(7);

commit;

update results set a = (