"Never mind, I was just thinking out loud," Erin said. "What I meant is, make the assumption that Earth and Equestria operate under a different rule-set. The difference is very slight, but it's enough to confuse matters. Maybe it's the lack of a magical field on Earth, maybe it's something completely different, but still... If they're designed to recognize and restore harmony to Equestria, then it makes a lot of sense that they wouldn't work anywhere else!" 