What effect does the dramatic twist have on Framton and the reader in "The Open Window''?

With her self-confidence and poise and the use of the open
window as the framing (pun intended!) device for her tall tale, both Framton and the
reader are taken in; there is no question. Thus, believing that Vera's narrative is a
tragically veritable history of Mrs. Stappleton's family, Framton, along with the
reader, feels pity for the deluded Mrs. Stappleton when she exclaims, "Here they are at
last!" 

But, of course, it is not Mrs. Stappleton, but
Framton, along with the reader, who are deluded. Poor Framton, is terrorized by the look
of horror on the girl's face, for he does, indeed, believe he has seen ghosts, while the
reader, who has the benefit of the omniscient narrator who describes the men as they
approach the house, is merely a bit disgruntled at her own guillibility, yet certainly
appreciative of Saki's wonderful use of irony and the surprise ending. As a sort of
apology for fooling the reader, Saki adds the wryly humorous last sentence: "Romance at
short notice was her specialty."