Gold Difference Pre-Pickpocket vs Post-Pickpocket

Maybe Skyrim inhabitants are in the habit of carrying a small amount of gold in their pockets, but keeping their main gold stash in a location from which it is much harder to steal, e.g. in their boots, hidden pockets sewn into clothes, in their underwear etc. Once they are dead it would be possible to retrieve this gold by doing a thorough search.

Are you an Imperial by any chance? It might be that your "Imperial Luck" ability works on corpses, but not on the pockets of the living. I don't know that anyone has ever looked into this, as the piddling amount of extra gold you get is pretty much insignificant.

I'm not imperial and I can confirm I see similar things. The 'loot' gold found on a body is based on your level and possibly other attributes which doesn't come into play until the target is dead.