How different valued coins were abstract back then

ANKH, first game I both played and owned. Currency was in three different coins. Ten copper coins equals one silver coin. Ten silver coins equal one gold coin. Really, really simple. But what made it a bit funny and abstract when I think about it now.

Obviously, if something cost 2 silver coins and 5 copper coins and character didn't have change, he payed three silver coins and got 5 copper coins in return. That makes perfect sense. But one thing was bizarre. If character had 8 copper coins and got 5 copper coins more, magically those 13 copper coins transformed into one silver coin and three copper coins in his purse.

We marked currency down adding them together in coins and if there was enough that it would be more valuable currency, it transformed to it!

If character had 2 gold coins, 8 silver coins and 9 copper coins and he got 12 copper coins wouldn't it be logical, that he now had 2 gold coins, 8 silver coins and 21 copper coins? Yes, but our logic told character now had:
1 copper coin, 0 silver coins and 3 gold coin!

How silly. Too much math in school I guess.