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.
Ok I stand corrected VOY did use one of the previously established figures.
However in TMP we are told that they could have Spock back on Vulcan in 4 days. A distance of some 16ly.
So at 4ly per day = 1460 ly / year. Which gives a journey time of around 51.4 years. Though we don't know what speed that is at. It could me the maximum speed ~Warp 7 (TOS scale), or Warp 5 saying the dailouge seemed to indicate that Kirk wanted to give the Enterprise a proper shakedown cruise.
But in any TV/Film production, any vehicle travels at the speed the plot needs it to. Even if we've seen it travel faster.
And on a more scientifc note it is physically impossible for the GQ to be 70 000ly from Bajor, given that our galaxy is ~100 000ly wide.
But back in the world of ST, The GQ terminus of the wormhole however is ~ 70 000 from Bajor.