What is the subtle difference between "and so on" and "and so forth"?

Kurt Vonnegut uses "and so on" a good deal to end his sentences in Breakfast of Champions.

In some places people would actually try to eat mud or such on gravel
while babies were being born just a few feet away. And so on.

Of all the creatures in the Universe, only Dwayne was thinking and
feeling and worrying and planning and so on.

It said to simply anybody, in effect, “Hey—guess what: You’re the
only creature with free will. How does that make you feel?”
And so on.

He would get down on the floor and roll around with Sparky, and he
would say things like, “You and me, Spark,” and “How’s my old buddy?"
and so on.

He would kid Bill about that. “How’s the old respiration, Bill?” he’d
say, or, “Seems like you’ve got a touch of the old emphysema, Bill,”
or, “We never discussed what kind of a funeral you want, Bill. You
never even told me what your religion is.” And so on.

The expression was first used by news photographers, who often got to
see up women’s skirts at accidents and sporting events and from
underneath fire escapes and so on.

“Vern’s wife thinks Vem is trying to turn her brains into plutonium,” said
Dwayne. “What’s plutonium?” said Harry, and so on.

My editor once changed my "and so on" to "and so forth" in a story.

Why?