Or then it's the small but significant difference between men and women on Vulcan...
In "That Which Survives", it's stated that 11.33 hours at warp 8.4 would cover 990.7 light-years. In dialogue involving Spock.
It's one of the worst outliers, yes. So perhaps we should nitpick it more carefully than any other?
The distance of 990.7 ly is not actually stated in connection with the speed and the ETA at that speed. It is stated a bit earlier on, making it possible that some of that distance had already been covered.
Of course, the only intervening scene there is one of our stranded heroes having at most half an hour of activity. And there isn't much activity after the scene ends but before the ETA is given, because we know this activity consists of making a memorial for D'Amato, and our heroes set to work at the end of the scene and complete the work soon after the ETA scene.
There might be activity before the surface scene but after the Rahda-and-Spock-discuss-distance scene, of course. Perhaps our castaways spent a few days assessing the available resources before accepting that they had nothing; that sounds like the human thing to do, really.
If the two shipboard scenes are separated by days, then the speed corresponding to warp 8.4 is reduced by about one order of magnitude. And if we remember that Scotty promised to give more than warp eight, we can speculate that he gave warp ten for the early part of that multi-day journey, and the ship was only forced to slow down to warp 8.4 just prior to the second scene.
That is, certainly this scene involves a speed change, otherwise the speed would not be discussed there - but we don't know if they accelerated to warp 8.4 or slowed down to that. The latter is eminently possible and helps reduce the speed of warp 8.4 all the way down to VOY-compatible numbers if necessary. It just requires us to think that the ship did warp 10 or so earlier on, and that our surface heroes survived without local food or water for a number of days (which should certainly be possible with the help of McCoy's little black bag).
Timo Saloniemi