Thank you all for the answers!!

Quote Originally Posted by The Darkhorn
Did you have a governor? I suppose he could'vd gotten some terrible vice that severely lowered his acumen.
I have governor in this settlement, I haven't changed anything in this settlement, only pressed the End Turn button.

But accidentaly I've noticed something interesting - I've saved the game just before ending the turn - the base farming level of the forementioned settlement has 240 denari income. And when I add recruitment of the Town watch in another settlement then in the next turn the base farming level of this first settlement changes to 160!! but when i don't do anything and just press the end turn button then the farming level stays at 240 (it of course sometimes changes by +/- 5% due to harvest changes poor/average/excellent but the base farming stays). is it a bug or I'm missing something? thanks in advance