Why does contraband or anything else change at all? This is an agrarian economy that moves at the pace of a person walking. It should take at least a year before anything changes at all.
What that line says is that from the time you talk to the smuggler in the tavern to the until you walk back to your ship the contraband will not change. That is some guaranty when it only takes 1-2 hours to do that. This implies that the economy changes hourly. I'm sticking with the old beta 3.3.
The contraband changes a bit. When a island decided to change the patrolling state a piece of contraband can be added or removed depending on the state. It wont happen for at least 120 days (this can be changed), and often will take way longer then that.
Normally in the time it takes you to do the smuggling mission it wont happen but just to be sure its added. Cause it could be the last time the contraband changed was like 300 days ago and you have a smuggling mission during the night. the change happens with the daily update, so there is a really small chance of it happening. thats why its added.