In case anyone spots a similar issue:
My problem specifically was that certain updates to a course settings would result in the previously configured ID values when adding a calculation were replaced with something like ##gi83##.
Through a bit more digging i was able to establish that although the ID i had set had disappeared ##gixx## was still pointing to the same item in the mdl_grade_items table and thereforethe calculation was still correct.
Hope this helps someone else in understanding this.
mathew