That happens to me All The Time. My analysis of the situation (being who I am, I must analyze it) is that it's my subconscious nibbling on the existing facts after writing the whatever, rather than my subconscious putting in the whatever knowing that it will be needed later. Putting together patterns from sometimes-apparently-unconnected information is what we do, we ape-derived sophonts, and writer-types tend to be better at that than most (so do scientists and detectives). Why should we not be surprised when our mystery-solving brains manage to solve a mystery, even if all of the pieces were provided by that same brain and we didn't even know that it was a mystery until the solution appeared?