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.

**Source**

Hope I helped.