[s1mplicissimus]

I'll try to express this as a programming analogy:

You are correct that to the outside the EU market negotiates as a single entity, like, say, an interface:

  interface Market {
    interact(string payload)
  }

on the inside though you will quickly realize that the payload format is XML in one country, Json in another, a completely custom format in a third. And that's not even talking fields, names, types, semantics etc. Hope this could clarify the confusion.