"It's possible that different planets or solar systems have different harmonic properties and this one in particular just happens to have the property of convenient flatness, but it's also possible that there are multiple mortal worlds in this picture, and if that's so, maybe there has never been a gate to a different planet in this - galaxy, universe, what have you. I suppose if you like you could make a test gate to Earth and test its harmonic properties - Earth is definitely the strongest candidate for 'planet in this galaxy that fairies have been to before' - but I'd still prefer to soothe my worries about gate locations relative to planets-in-general first."