Likely the heat of the water melts the butter to some degree and makes it blend in better. I can see it with the edible oil products like coffeemate and they were originally designed to blend in and not separate and perhaps the additives have an effect also. Perhaps because of the amount of shortening added too. It is interesting, but if you think about it, when you grease a frying pan, heat it up and then put droplets of water on it, they bounce away from the grease, like the old saying about oil and water not mixing.
Interesting about the warmed water making a smoother icing. I don't know if you add salt or meringue powder to your icing, but I would think this is a factor too.
It is like using a high ratio shortening as oppposed to Crisco. Well, the high ratio shortenings can absorb the sugar better, of course the finer your powdered sugar the better. But this is what makes for a smoother icing. Of course if you use a really good high ratio shortening and then use a poorer quality powdered sugar, one that is not fine enough, well you are going to lost the effect.
I can tell you that when you make a drizzle from water and sugar or milk and sugar, you don't have a problem, but introduce softened butter to it, not melted, but softened, and it curdles, or appears curdled.
Actually I do find a big difference in taste, even when milk is used as opposed to cream etc. Maybe some people are more sensitive to taste, I don't know. I know that if I use all cream instead of part cream, part whole milk, I can taste the difference along with see a difference in texture. If I sift my icing sugar before using it as opposed to using it unsifted, I can feel a difference in the texture too.
Hugs Squirrelly Cakes