How does "selfsame" differ from "same"?

In 2002, I asked Merriam-Webster the following question. Editorial Assistant, Ben Korzec, replied as follows.

Q: How does “selfsame” differ from “same”?

A: Our dictionary lists “same” and “selfsame” as synonyms, so you can use them interchangeably. Not only do they have the same defining cross-reference to “identical,” but they also have similar senses.

“Same” means “being the one under discussion or already referred to,” and “selfsame” means “being the one mentioned or in question.”

The verbal illustration “left the same day” also illustrates selfsame: “left the selfsame day.”