It is fairly simple.
Let's posit that there is a standard marker on a table in an otherwise empty room. Now, lets say that there are four people in the room. Each person will see the room, each other, and most importantly the marker (the focal point) slightly different or very different depending on their physiology. Each person is focused on the same marker (ultimate reality) but are "seeing" something slightly different as if they were witnessing the same thing through different prisms or glasses. Each person's interpretation of the marker (ultimate reality) becomes their own "personal reality".
The problem with your post, and extreme rationalism in general, is that it negates the power of experience. At some point, even the most stringent rationalist must concede that one's concept of personal reality is rooted in empiricism (experience). You are correct in asserting that one will never truly know or understand ultimate reality; one can only have a perception of ultimate reality. However, only being able to have a perception of ultimate reality due to physiological limitations is not a logically valid reason to dismiss the concept or idea of an ultimate reality outside of a thinking being (yourself).
After all, where does one get their concept of reality if there isn't an "original" reality from which the observer draws their perception from?