I have noticed the same bug. Gold value of each coin, when NPC has 13 coins, is 13 gold for the first coin and 12 gold for the secon etc. When you steal all 13 coinst at once game shows 169 gold. In this particular case (pics on screenshots) it's more profitable to steal 1 coin at a time (91gold vs 169 gold).
(screenshots with bug)
http://steamcommunity.com/profiles/76561198302046637/screenshots/?appid=0&sort=newestfirst&browsefilter=myfiles&view=imagewall&privacy=14