Board Thread:Arrow discussions/@comment-9346948-20171128022612/@comment-24152851-20171212104444

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.