I am a collector of sovereigns mate and I think what you are seeing is wear and tear. Being 22ct gold sovereigns are very soft and wear very quickly and as high edges become worn they change. You would be better off comparing your coin to a coin in similar condition, and don't compare to a coin in UNC or aUNC. I am not a coin grader but your Half sovereign would struggle to be fine and when it comes to gold coins there is a long way between fine and even very fine never mind unc, and these coins change quite a bit through those grades. And a lot of this is how they grade them looking for areas on the coin showing wear, and how much.