First the WTS said it was 606 BCE.
Then someone pointed out to those alleged chronological experts that there was no year 0 (the calendar went from 1 BCE to 1 CE).
That meant that the year should be 1915, not 1914 (2520 - 607 + 1 = 1915).
So the WTS changed the starting date to 607 BCE to make the math work out to 1914.
By the way, if the "Chronology" section of the Aid to Bible Understanding book seems defensive, it is. In CoC, Ray Franz reveals that he wrote that section after realizing that there was no evidentiary support for the 607 date and no apparent motive of scholars to promote the 588-586 time frame instead. Still loyal to the WTS in those days, he spilled a lot of ink trying to convince the reader that evidence yet to be uncovered might support the Watchtower's claim.
That was one of the facts that led to his crisis of conscience.