You don't believe in the warp recalibration?
How Kirk could fly at warp 14 (Warp 36 if you count TAS), but warp ten is infinite speed to Picard, however futureboy Riker is zooming forward at Warp 13 in All Good Things.
It's certainly a possibility that the Warp Scale could have been recalibrated between Archers time and Kirks as well?
Leagues, Knots, mph, kph, mach numbers, fractions of C... Multiples of the Speed of light?
It happens all the time.
How do you know when you're dealing with a leap light year and not an ordinairy light year?