[Arthur's just such a self-sacrificing guy, welp. It's true, though. And... he knows he sucks at explaining all of this. asdhgasd; Merlin should have been the one to find him instead. ... Not that he doesn't doubt Merlin will find Lancelot soon. He'll be happy -- right?]
That's how it supposedly works. This land has a powerful magic, that will draw in those from any moment of their lives. Merlin is... several weeks past me. And there are others here, from different lands and times. I don't fully understand it myself, but most just say it's the work of the magic here, of the land itself.