It's natural that there'd be a bit more ambient magic energy than average in Moperville given a steady flow of magic to it, but it's not supposed to just stay there. Which, technically, it's not doing, seeing as it's spreading out the worse this gets.
I've been waiting ages to bust out this coin analogy, which I'm hoping helps make things pretty clear. For most intents and purposes (sorry, Andrea) the griffins are from another world, but not a totally separate universe. There are things in one that can impact the other.