I am taught that a prophesy often is two-sided :) At first, the prophesy is relevant for the person, this is called in my language "obvious/upper layer", and there is often a second layer, that is universal and for the future. For example the prophesy of the virgin birth, which is originally for the Jewish king back then, but also a sign of Jesus birth. This is quite common. So, I think David didn't know he was prophesying about Jesus, that he did feel that at the moment, but because of the Holy Spirit that was present in him, he actually prophesied. Also keep in mind that though crufiction was unknown back then, David describes a bit of that, too. Hope that answerd your question!