It's the fact that it's not a junction table, it's an association table - i.e. it has additional business info (fields), including one that is part of the composite pk. (i.e. it's "transactional" and not "transparent")
So (in my case), you have 2 fk's, both of which are part of the primary key, along with one of the "extra" fields. Now, since pk's are unique, don't you end up with two on-to-one relationships due to the unique constraint being applied to the fk's? Is that what you want out of a "many to many, plus data" relationship?
Thx!