That's a good question. We have several options. It is possible that during a hunting party the hunters lost something too
. Trading is also a good posibility because the blades are mostly javanese. Sumatra was known for trading throughout the archipello. And javanese blades were highly apreciated on Sumatra. So there was delivery of javanese blades to Sumatra. These blades were used and mounted as Sumatra keris and I can imagine that the surplus was used for trading.
Just some opinions, because i don't know the answer either.