Butter(-milk)

I recently used the Joy of Cooking's recipe for Irish-American Soda Bread as the foundation of a kitchen experiment. JoC is usually a great reference and an excellent starting-point for me. I rarely have all-out success with their recipes as-printed, but who wants to follow a recipe verbatim anyway?

What caught me this time was the following ingredients:

4 tbsp butter
2/3 c. buttermilk

The butter was meant to be melted and whipped into the buttermilk with an egg.

Now, buttermilk is the by-product of whipping cream into butter. So by adding melted butter to the buttermilk, aren't you just re-constituting the original heavy cream? Even the proportions look about right here.

So why?