I personally like the purer gold, but they sent you that likely because they had too many in stock and were looking to reduce how many they had laying around. It's very unlikely it was an error. It's not a bad looking coin, I might keep it just for numismatic reasons. Either way you'll end up with a nice coin for sure. Sometimes the coin of their choice will be worth more than you pay just because of what they have overstocked!
I never thought of that, but you might be correct.