^^That's just one instance of a concept that's been around for quite a while. Given that the supposed "official" warp formulae published in behind-the-scenes material have never corresponded to the (generally much higher) apparent warp velocities used onscreen, and given how wildly inconsistent the distances and travel times onscreen are, it was necessary to formulate the idea that warp factors are actually measures of power usage and that the actual velocity corresponding to a given warp factor varies depending on the conditions of local space -- mass distribution, energy concentration, subspace handwavium levels, etc. The earliest work I'm aware of that mentioned this is the Introduction to Navigation booklet accompanying Pocket's 1980 Star Trek Maps, which referred to this fudge factor as the Cochrane factor.
But it wasn't just used in fandom. In the TNG Tech Manual, it states the same idea, that actual warp velocity varies according to local conditions. And the warp table published in the ST Encyclopedia (reprinted from the Writers/Directors' Tech Manual) says that the table only gives approximate values and "your actual mileage may vary." (Yet oddly, most people in Trek-tech fandom seem to ignore these caveats and expect the warp factors in those tables to represent exact and consistent values.)
The reference in Star Charts is to the idea that sometimes these varying conditions form "space lanes" that allow much higher velocities than usual. This is often used in fandom -- and arguably is necessary -- to explain some of the more extravagant "shortcuts," like the trips to the galactic rim and center and the 4-day trip to Qo'noS. And it helps explain why DS9 could be only days from Earth when we didn't discover its neighboring worlds for centuries -- because it wasn't easy to reach that area until the Federation found a suitable "space lane."
Indeed, it's even possible that warp travel helps create space lanes by modifying subspace in a way that "smooths the ride" for subsequent ships following the same approximate path. So frequently travelled routes might become quicker. At the very least, in well-charted regions of space, it would be easier to know what the fastest route was, whereas in new territories, it would be slower going. I've alluded to the latter idea in several of my Trek novels.
