[Zardoz84]

The TNG scale have a asymptote at Warp 10. The TNG scale come from a draw, and actually have a very complex formula derive from these hand draw graph, but can be approximated to a exponential scale below Warp 9.

TOS (and Enterprise) uses other scale with a much more simpler math exponential formula, were the same Warp factor is less quick that in TNG scale.

Actually the TNG scale is because Rodenbery like to give the impression that the ships are more fast that in TOS but he like to avoid using Warp 10 and huger numbers.

You can check it in http://en.memory-alpha.org/wiki/Warp_factor , appendices section.