In the mind's eye

Calvinism as well as some varieties of freewill theism (e.g. Molinism, classic Arminianism) affirm God's future knowledge and/or counterfactual knowledge. However, even in that respect they share less in common then meets the eye.

Let's take a comparison. I've read that Alfred Hitchcock filmed his movies in his mind before he filmed in reality. We might say he had a mental representation of the future film. Likewise, if I watch Psycho, my memory of the film is a mental representation of what I saw. 

Yet there's clearly an asymmetry here. On the one hand, my mental representation of Psycho is a copy of the original whereas there's a sense in which Psycho is a copy of Hitchcock's original idea. He filmed what he saw in his mind's eye, whereas I visualize the end-product. What's in my mind's eye is caused by the film while what's in his mind's eye is the cause of the film. 

That's analogous to the difference between Calvinism and freewill theism respecting omniscience. In freewill theism, God's knowledge of a creature's future or hypothetical actions is a copy of what they will do or might have done. For their agency is in some respect independent of God. In Calvinism, by contrast, a creature's future or hypothetical actions is a copy of God's exemplary idea. 

In Calvinism, God's concept of what will happen or might have happened is metaphysically prior to the outcome. That's the source of origin. The mental event (in God's mind) is primary while the extramental event (outside God's mind) is secondary. 

In Molinism or Arminianism, by contrast, what will happen or might have happened is metaphysically prior to God's concept. His concept is derivative. The extramental event is primary while the mental event is secondary.

In Calvinism, what will or might have been is ultimately subjective to God. Originating in God's mind and will. 

In Molinism or Arminianism, what will be or might have been is objective to God, originating outside God's mind. His mind mirrors that semiautonomous reality. Imprinted on God's mind.