We, for instance, can't know or measure the precise state of things as they would've been had something different occured than what actually occured at an earlier time.
This is especially obvious in the realm of quantum mechanics, where you can't know what you would have measured if you had measured earlier or later than you did (I hope my interpretation as a layperson was correct here). That information just doesn't exist in our reality, but it may exist in some higher order place where all those realities can be observed.
These attributes can't have any effect on the phenomenal world, or they'd be detectable, and by definition, they're not detectable by us. Unless, of course, our free will is part of the noumenal world, and our actions in the phenomenal world are simply one way of viewing our will.
We're trapped in our brain, behind a wall of sensory apparatus. There's no reason to assume that our senses provide the complete totality of reality. And if there are aspects of reality we cannot sense, then there is more to reality than we can ever know. Kant called these attributes the "noumenal" world, while what we can see and observe is the "phenomenal" world. However, these are NOT separate areas - just two aspects of the same thing. Not dualism.
Schopenhauer took this to the next step and said that there are aspects to ourselves we can't observe, so part of our selves exists in the noumenal world. The phenomenal world is deterministic, but if we have free will, it must exist as part of the noumenal world.