Subquery Returning Multiple Rows

I understand what the message "subquery returning multiple rows" means but I have a case where I'm not 100% sure why it's happening to my update query (which in turn probably means I don't fully understand what's going on behind the scenes)

Here is my query:

Update A set (A.id, A.alt_name, A.min_rank)=
(SELECT B.id,
B.fullname,
MIN(B.nm_rankval)
FROM B,
A
WHERE A.id = B.id
AND A.name <> B.fullname
AND B.nametyp = 'ON'
GROUP BY B.id,
B.fullname)
;

The subquery returns 6 rows but they are all unique in terms of the id, name, rankval, etc...I naturally thought that the update statement wouldn't have a problem with this since the subquery is returning rows that are not duplicates and match one for one between table A and B. I used the group by to ensure I return unique values from table B (which does have duplicate id values)

Each of those 6 rows from the subquery of table B can be matched 1-1 with table A...so what am I missing.