I think this is why it's hard sometimes to argue in support of something you believe, even if you're right. At one point, all of the relevant facts and figures were loaded into your working memory, and with that information you arrived at a conclusion. Your brain, however, no longer needs those facts and figures; you've gotten what you needed from them, and they can be kicked out of working memory. What you store the…
I often say "X was explained to me once and it sounded reasonable, but I don't remember the details anymore."
Sometimes remembering the reasons themselves for X off the top of your head may not be important, but knowing that there are reasons (that you can look up) is.
What the answer is for something may not be as remember as remembering that an answer exists.