> > Twenty persons sat down for dinner.
> > Twenty-one left the dining table.
> > How come?


How about this: one of the twenty people was a woman in the last stages of pregnancy. She unexpectedly went into labor a little early, and there was no time to get her to a hospital, so she ended up giving birth right there at the dining room table...hence, 21 people ended up leaving.

Or: one person stood up to have dinner (perhaps they were short on places to sit?).

Or: the family dog was named "Twenty-one", who had to leave the dining room table when the 20 people were ready for dinner.

I'm sure there must be other possible answers too, but I'm betting on the pregnancy one as being the one you wanted.