On the Memory Alpha page for All Good Things, this is mentioned - bolding mine:
- It is clear that there is no more warp 5 limit in the future. This limit was set in "Force of Nature". In addition, ships in the future timeline are able to go above Warp 13. This would appear to contradict "Threshold", where it is stated that Warp 10 is the theoretical limit. It is, however, possible that the warp scale was recalibrated in this future.
This is a reference to the different scale used in TOS compared to TNG. During the TOS era, there wasn't a limit. In-universe, at some point between the two settings, the warp scale was recalibrated, giving different speeds for each warp factor.
TOS-era (TOS+ENT), this was the official calculation for warp factor:
In the TNG era (TNG+DS9+VOY), the recalibration had two parts - for warp factors below 9, this was the equation:
With Warps 9-10 scaling up exponentially, asymptote at 10.
The TNG curve was based on supposed in-universe physics, where the thresholds for Warp 1-9 were based on some sort of transition that involved extra power usage. Warp 9-10 spanned the remainder of all warp speeds because there were apparently no further spikes of that sort:
Thanks to a related question, this image comes from Michael Okuda, art supervisor for TNG and beyond. He created the new warp scale. The curve from 9-10 has no definite equation because it was based on a hand-drawn curve. (Wikipedia)
So these are the maximum warp speeds available in the TNG era:
- Enterprise-D (Galaxy-class) - 9.8
- Voyager (Intrepid-class) - 9.975
- Defiant (Defiant-class) - 9.982
And since technology was only getting better, these maximum warp speeds would have steadily gotten more absurd, since Warp 10 ("Infinite Speed") isn't actually possible under that scale (Threshold excluded).
Another recalibration was inevitable, with All Good Things hinting that they returned to a TOS-style scale that didn't have an upper limit.
Ex Astris Scientia includes some additional information:
There is no official formula to calculate the speed corresponding to the revised warp factors. In the October 1995 issue of the Omni magazine, science advisor Andre Bormanis states: "I raised that question in a TECH note. Basically, the idea there was that they recalibrated the warp scale. I don't think that ended up in the final draft teleplay, but the idea there was that if you've got ships that can routinely travel at speeds in excess of Warp 9, then maybe it makes sense to recalibrate your speed scale so that Warp 10 is no longer infinite velocity. Maybe Warp 15 will be the ultimate speed limit, and Warp 13 in that scale will be the equivalent of warp 9.95 or something like that."
The page when goes on to basically theorize the in-universe reasons, which are essentially what I posted above.