Whatever they said, Warp 9.975 is NOT the maximum sustainable speed of an Intrepid-class starship. It’s the maximum possible speed… for a few hour at best, before the warp core needs to be powered down to prevent everything from melting.
It’s not a stretch to assume that the initial 75-year estimate was based on average sustainable speed and taking into account time needed to stop, refuel, maintain, repair, etc. Over that long a time, it wouldn’t be enough to just pour more deuterium into it. Most of the warp engine would have to be rebuilt or replaced.
Also, the computer was struggling to figure out exactly where they were, so it might have been off by a few years at first. When Seven built astrometrics, they immediately updated their ETA.