Board Thread:Arrow discussions/@comment-9346948-20171128022612/@comment-2054358-20171213023201

Stryzzar wrote: Krypton has to exist on Earth-1 though. It just seems like a stretch for Cisco and Barry to happen to come across Kryptonite on an offscreen adventure on Earth-38, take it back to Earth-1, giving it to Oliver, who happened to decide on making it into a trick arrow.

It's more likely that Oliver was prepping for a Kryptonian battle and went to track down Kryptonite on Earth-1. Onscreen, Oliver didn't even know about Kryptonite until the moment he shot it. So Barry would have to tell him about it offscreen. And if he told him offscreen, he could give him the Kryptonite from Earth-38.