From the introduction:
Why Flash Fiction, as opposed to Sudden Fiction, which we have featured in two previous books? Answer: We did want to make a distinction between the two types of stories. The stories here are shorter (in terms of "limit") by a full thousand words than the stories in those books, and quantitatively there is a big difference between 1,700 and 750 words.
One of the original ideas for the book was to present stories that could be read without turning a page, assuming that there might be some difference in the way we read stories when we can actually see beginning and end at the same time. So, envisioning a story on a two-page spread, 750 words seemed about tops for conventional, readable typography. Enthusiastically, we began searching for such stories, and called them "flash" fictions because there would be no enforced pause in the reader's concentration, no break in the field of vision."