There's a few logical possibilities here.

#1 is that whatever stimuli produced it the first time keeps producing it later.

#2 is that you clearly did remember them enough, possibly when you woke up, to commit it to long term memory. This is a tautology, because you would have to remember it to know that it was in previous dreams.

#3, as someone mentioned below, is your mind playing tricks on you. Ever had a dream where you knew an object was something important and specific, even though its appearance in the dream was clearly not that? E.g. you know you're at your (specific) friend's house, but that's not at all what their house looks like. Dreams have a lot of weird substitutions like this, including the idea that you've been somewhere before