The 1920s was "a long time ago" and the Andromeda galaxy is "a galaxy far, far away." Maybe the original trilogy took place 100 years ago in the next galaxy over.
The Enterprise D Technical Manual that Sternbach and Okuda released, which was supposed to be about as canon as it gets, says warp 10 is infinite speed. Go figure.
Maybe it's Warp 9.9 that is ever increasing. I don't really remember, but I distinctly remember reading two different tech manuals describing the "fastest possible speed of any federation vessel" as two entirely different multiples of C, and Warp 10 being defined as the fastest possible speed.