Because they are sure the Gentile Times ended in October 1914 but not how soon after that date Satan was hurled down from heaven: a few days, weeks, months, years?
The hurling down of Satan is the event described as "about" 1914, not the actual end of the Gentile Times in September/October 1914 itself.
Not that I agree with any of that chronology, I'm just pointing out that the use of "about" indicates no new sense of wavering over the correctness of date 1914 in their chronology from a JW point of view.