9.975 Might have been the "Top Speed" listed on the Ship Specs.No vehicle, be it car, boat, or starship, is capable of maintaining maximum velocity at all times. Warp 7.5 is pretty respectable, in my opinion, for Voyager with a top speed of 9.975. I'm surprised it was that high an average with all the backtracking and side missions Voyager undertook in addition to scrounging for food and supplies.
But they also stated that 9.75 was sustainable for 12 hours, so they probably couldn't sustain 9.975 for that long of a period.