CorporalCaptain, there has long been a rift in TREK fandom over certain technical pronouncements made by Gene Roddenberry and others in Paramount. One concern was the meaning of warp velocity. In the publication
STAR TREK MAPS (Bantam, 1980), author Geoffrey Mandel put forth an expanded meaning of TOS warp technology with seemed to bridge the continuity throughout TOS, including TAS and TMP. The specifics of this publication tied into technical advice Roddenberry et al. received from NASA manager
Jesco von Puttkamer, as published in THE MAKING OF STAR TREK: THE MOTION PICTURE.
A booklet included in MAPS entitled "Introduction to Navigation: Star Fleet Command" listed all the worlds explored by the U.S.S. Enterprise during TOS, as well as other key worlds mentioned in TREK. The booklet also provided a detailed monograph on the history and technical workings of warp drive. Part of this monograph included a detailed table based on Cochrane's Formula for calculating real-space FTL velocities based on warp factor that included a heretofore unheard of element: Cochrane's variable. Cochrane's variable, indicated by the use of the Greek letter chi (looks like an italicized "x") which enhanced warp speeds based on environmental factors.
Cochrane's formula formed a basis for logically explaining how warp velocity could be so incredibly fast in some situations (traveling over 1,000 light-years in a day; "Obsession", "That Which Survives") but then warp speeds seem to drop to their "ideal" values (chi = 1.0) in others. When TNG started up, Roddenberry et al., put in place the anti-FJ rules, and MAPS became a "canon" casualty. Oddly enough, there are examples of TNG warp speeds resembling the same variations as what MAPS tried to capitalizes on in TOS/TAS/TMP (off the top of my head "Conspiracy", "Where Silence Has Lease", "Tin Man", "Transfigurations" and "The Best of Both Worlds" seem to fit) but Roddenberry and company refused to acknowledge the issue. Fans of MAPS who picked up on Richard Arnold's pronouncements of certified "Star Trek fact", another way to saying what's canon and wit isn't, were a little miffed.
So if you put Mandel's notion of warp drive and velocity to work on TOS, TOS velocities can, in some circumstances, appear faster than the Roddenberryan TNG Warp 9.999 asymptotic curve approach. If, on the other hand, you retcon Mandel's approach onto TNG and later series, you can make the argument that warp velocities are more fluid, like sailing ships calculating trade winds.