The facility he worked in refined the gold that was shipped to them, so they end up losing quite a lot of mass when the impurities are removed. But why they wouldn't measure the output to make sure it was at least close to the input, is kind of a stupid thing on their part.
But what really confuses me, is why didn't the guy bother to melt the pucks down into a different shape. Maybe make them look like some old colonial-era coins or something that he 'found' while diving or something. Hell, might be able to fool coin collectors and get even more money out of the gold.