That is a good question to which I would like to know the answer too as well.
In Australia today at a major bullion company they are selling the following one ounce items.
1 oz SILVER coin - 2017 Australian Kangaroo 0.9999 - Perth Mint $26.18
1 oz Silver Bar - 99.99% Minted Silver Kangaroo Bar - Perth Mint $26.27
1 oz silver coin - Sunshine Mint round 99.9% pure $25.68
The American round is cheapest. But in this case the Kangaroo bar is the dearest and more than the legal tender Silver Kangaroo coin. Why would the bar be more expensive than the coin?