In BlogPost.php, the canEdit method immediately returns true if its parent's canEdit method returns true.
if (parent::canEdit($member)) {
return true;
}
This happens before any of BlogPost's bespoke checks (e.g. checking $parent->isEditor, $this->isAuthor, etc) which ultimately results in some members being able to edit any blog post even if they're not an author for that post (depending on the permissions checks up the line).
It also means that if the parent says a member should not be able to edit the page, if someone else has listed them as an author they may be able to edit the post anyway.