Why the figures differ
An insurance valuation, probate valuation, retail asking price, auction estimate and direct buying offer answer different questions. A direct offer is not a fixed percentage of an insurance valuation. It depends on tested metal, stones, condition, demand, fees, authentication, market liquidity and the route most likely to produce a real sale.
Auction can be better for rare signed, period or collector pieces. A direct specialist offer can be better where certainty, speed, item-by-item decisions and avoiding seller fees matter more.