In his siddur, Rav Sa`adya haGaon (Rasa"g) mentions only four of the five species of peroth ha-ilan which are special to Eress Yisrael, leaving out zethim ("olives"). He does so in two places. Further, he does not simply list the other four and leave out olives, rather he precedes his listing of them with the words huss me-arabaah dhevarim ("except for these four things"). This seems to indicate a volitional (i.e. shitah-based) reason for doing so.
Question: Why does Rasa"g not mention zethim in his list of the five peroth ha-ilan which Eress Yisrael is known for and which require Birkath Me-En Shalosh (i.e. al ha-ess we-al feri ha-ess)?