You know, I've found myself doing the same thing too. Maybe, it could be where we don't actually recall the dream right when we wake up but later on when you dream about it again that it reminds you...hey, i've dreamed this before.

Sometimes I won't remember a dream right when I wake up, but later in the day I'll see something that triggers that memory and say HEY, I HAD A DREAM ABOUT THAT LAST NIGHT!

and maybe an idea is that if it is a reoccuring theme, then it could be a dreamsign. Idk, just a couple of thoughts!