What is necessary is necessarily eternal, but the eternal is not necessarily necessary.
Time – which is to say, congeries of contingent events, that are causally related and that therefore, together, constitute worlds, extensive continua along time, space, and myriad other dimensions – occurs in eternity. It occurs eternally (and only then, and only in virtue of its eternal occurrence, temporally), but not necessarily. It occurs freely. So likewise also for God’s Act.
Eternal acts can be free. They are not necessarily necessary. Some may also be temporal, such as this moment in your life, or the Incarnation.
Necessities comprise what Whitehead called the Primordial Nature of God, and Plato the Realm of the Forms: the Nature in virtue of which there is such a thing as order in the first place, the order of all order. The free eternal Act of God, and all its derivates in his knowledge, comprise what Whitehead called the Consequent Nature of God. Both these Natures are eternal, and indeed coterminous, in that together they characterize a single Act; so that they are sections of a single Nature. But of the two, only the Primordial Nature is necessary.
NB: God’s omniscient knowledge does not continge upon creaturely acts, but vice versa. It is only in virtue of his logically prior knowledge of creaturely acts that creatures may act in the first place.