Specifically, the computer said "nearing maximum velocity." That is to say, the highest speed at which the engines are physically incapable of exceeding under any circumstances. Evidently, it cannot exceed this speed because any faster would cause them to immediately explode.
Check the dialogue again.
KIM: They're approaching Warp 9.9
CHAKOTAY: Increase speed to match.
COMPUTER: Warning. Nearing maximum velocity. Structural collapse is imminent.
CHAKOTAY: Are we in tractor range?
KIM: No. And they're still accelerating. Warp 9.97.
COMPUTER: Warning. At present speed, structural failure
in 45 seconds.
CHAKOTAY: Reduce speed to Warp 9.5. Keep a sensor lock on them as long as you can
Voyager's "maximum velocity" was determined by the ship's structure (the computer's warning of structural failure). It has nothing to do with the engines. As far as we know, Voyager could have hit Warp 9.9999 accept there wouldn't be much left of the ship due to structural problems.
From the series:
- Voyager has a stated top cruising speed of Warp 9.975
- Voyager has not been able to exceed Warp 9.9 in the series due to structural issues that probably did not exist prior to the series (thus the higher top cruising speed.)
- We have not seen a pre-series Voyager or another Intrepid-class attempt it's top cruising speed and fail
- We have not seen any deliberate deception regarding warp speed capabilities