It is true I used the common phrasing about assuming a DP rather than the definition in the rules. As you point out on a DP the rules clearly state that no error gets charged on the throw no matter how awful it was if the runners don't advance. This is a clear exemption to the rule that would give an error on a fielded ground ball on the exact same throw.
My question is why? It is a non sensical thing that two throws exactly the same without any outside impact would have two differnt scoring results just because one is tha back end of a double play. It's not the end of the world but just one of those things that baseball has that makes me scratch my head.