It is a contradiction with a clear reason for it:
If you were to take 6 days moving to prison, it would be boring and not fun, and add nothing meaningful to the game.
Think about it. 6 days on the way there, 7 days sitting in prison, and then another 6 days back to the capital—or instantly transported to prison, 7 days sitting there, and then 1 turn back to your capital.
Which would you really prefer? Particularly since your actual prison options wouldn't be available until you got to prison.
Well, you could modify it, to where prisoners travel like priests (say, 10 miles per hour), and have a chance to escape while traveling, and only sit in prison for 4 days instead of 7. Getting captured near to the enemy capital could result in less time in prison, while getting captured far away from the enemy capital results in more chances for escape.
The 7 days in prison is purely a game-mechanic rule, and can be changed.
