I experienced this with my older child. Ever since he could begin to talk he seemed to have exceptional memory, seemingly recalling things from even before he had words to describe them. But somewhere between 3 and 4 it's like there's been some culling of memories where a lot of things he would have known if you asked him on this third birthday he's seemingly forgotten by his 4th.
Not enough to be concerned of any cognitive things, and he still otherwise seems to have an excellent memory.
My child, at 8 or 9, remembered a game I used to play that scared her when she was 2 or 3. At 10 she could no longer remember the game.