Plain and simple the writers of VOY ignored any previous established figures, in order to make the jounrey seem longer than it should have been.
Not true. The "roughly 1000 ly per year" estimate used on Voyager
was initially established two years earlier, in the first-season Deep Space Nine
episode "Battle Lines":
The Gamma quadrant is seventy thousand light years from Bajor. It would take our fastest starship over sixty seven years to get here.
That differs from VGR's estimate (75 years to travel 70 kly) by only 12 percent, and that could be due to Voyager
not being the fastest ship, or Sisko failing to take stops along the way into account.