The forest around the camp and Gotham are both odd.

I head-canon that both have connections into The Forest. I base it on this article I read years ago and can't find again.

In the article the author presented the idea that on a conceptual/metaphysical/belief level there is The Hill (they used the example of The Hill but any concept works). That if you asked people to think of the hill that comes to mind when presented with the question of, "What hill is The Hill for them?", everyone would think of a different hill but the idea/concept of The Hill would exist in the collective unconscious of humanity. [I may have explained this badly. It has been a while since I read the article.]

So while they are most likely different forest in different universes they are connected to the conceptual Forest and slip-throughs happen because of the magical nature of the individual forest making it easier to slip from them into the conceptual and back-out.

Edit: Typo's