It's a miscalculation, mainly because Warp Speed factors were never meant to mean anything. Going at whatever is considered the average warp speed would take you 1000 LY in 1 year according to VOY. So 75,000 LY is supposed to be a 75 year journey at maintainable speeds (since they can't keep going at maximum warp for that long, they have to slow down sometimes).
So the only concrete thing is the 1 year = 1000 LY, and even that was sort of inconsistent with prior TOS/TNG stories.
Starships, like most vehicles, are probably capable of short dashes at high speed, but incapable of sustaining that speed for any significant length of time.
All right, so on two other occasions it was claimed that the ship could sustain warp 9.975 (or at least this speed was called "sustainable cruising speed"), which would be even faster than warp 9.9. But another episode clearly established that even one minute at warp 9.7 would tear the ship apart - so apparently "sustainable" is typical military-industrial bullshitspeak and means "sustainable for a few seconds"...
Voyager by the Threshold episode time had already operated at high warp for a Year and according to TNG extended trips at high warp require nacelle maintaince. Intrepid is a short range craft not meant to operate far from support.
But yeah the calculations were wrong.