I didn't find a source that says what chance you have to get an item so I just did the percentages myself, assuming no item had a bigger chance of appearing.
20% chance to find an item that is both in X and Y
14,3(approximately) chance to find an item that is only in X or only in Y
Since Old Amber is in both games, you would have a 20% chance to get it and I would say you're just unlucky.
Hope I helped.