Olives are bi-annual croppers and last year was a very good year. So when an olive tree has a lot of fruit it does not put on much new growth. It is on this new growth that the following years olives will be borne. So when you have a good year like 2013 then the following year will not be as good.
This is where we are. Do not worry if you have adopted an olive tree from Casa Margherita, you will still receive your two litres of single estate extra virgin olive oil.