The feature try to simulate a real case scenario where:
-The hunter first and foremost needs to get familiar with the area he is going to search for treasure. For this reason he must spend some considerable time. Once the hunter learns a region/port well, he will remember is for quite some time. This means that he can search anytime in the same port, without needing to re-explore. Of course, as much time passes, he will start losing memory but only real slowly (1% per region per week).

Captain Jack wrote:First of all, the manual mentions:
Your exploration rate drops 1% per region once a week.

This was never implemented. Therefore it has been removed from the manual. It can be revised as an option in the future.