The difference is that some believe God does indeed know all that there is to know, but exclude contingent events or outcomes as being among "all there is to know". They don't view God as less omniscient (e.g., God's omnipotence is not threatened because God can't make a square circle. Open Theists like Brian McLaren consider pre-knowledge of such contingent events along the same lines). Reformational Arminianists, like Robert Picirilli, view God as completely omniscient to all things occuring in time. God knows who will respond and those are the ones He elects. Although the mode of divine knowledge is the same in salvation (God's knowledge is dependent on the result of human free-agency), I don't think it fair in any way to speak of Open Theism as a form of Arminianism. One (Open Theism) does view God as having limited himself to knowing future events, or that future contingent events (apart from God effecting an outcome) are unknowable. But Reformational or Classic Arminianism does not.Except tht would see God as eally hving limited Himselfto knoingfuture ents, as he woulbe blind to tem until the happened in real time...
And Arminiasm still has ta pesky problem of Jesus did fo all, will ofGod to save all,, yet all not saved..
I also believe that Christ died for all, and even that God desires that all should be saved. And yet all are not saved.