They have different player populations. A pool of Elo ratings only rates players relative to the other players in the population.
If you take everyone whose FIDE rating is between 2400-2600, and you create a new pool of Elo ratings for them, their new Elo ratings will range from 1400-1600. If you take everyone whose FIDE rating is between 1400-1600, and you create a new pool of ratings for them, their new Elo ratings will range from 1400-1600. And so on.
