Problems about Set Type ?

[qeoshy qi]

I come with some problems when using set type.

eg:
create table T (col set('1','2','3','4','5','6','7','8') not null);
insert into T values('1');
update T right join (select 'aaa' as col) a on a.col='aaa' set T.col=8;
The result of T.col is 8 ???!!!

but it performance normal when
create table T (col set('1','2','3','4','5','6','7','8'));
insert into T values('1');
update T right join (select 'aaa' as col) a on a.col='aaa' set T.col=8;
The result of T.col is 4.

I wonder why it is 8 when I use 'right join' and 'not null'?