Jokes that, nowadays, need explaining

[Steve_MB]

@Couldn't get this 80s or 90s Reader's Digest joke Cafe Society

Sometime in the 80s or 90s, someone told a true anecdote that was published in RD’s joke section and I have never figured it out:

I was a pizza delivery guy. One time I delivered a pizza to a lady, and she gave me a $20 bill and told me to “keep the change.” Delighted at such a large tip, I took the bill and went back into my car, only to have her come and knock on the car window and clarify: “what I meant was, keep the change .”

Is there some hidden meaning here?

She meant keep the coins.