I would guess that the container they were buried in rotted away/collapsed (I'm thinking wooden chest?) and compaction of the dirt/earth would cause the pieces to look as they do today. After all, precious metals are soft.
Well, I guess that is a tip for the wise: when you hide your gold jewellry, hide it in a metal box that won’t collapse. Also it is significant that this jewellry is pure gold — no compounds to make it stronger.