Because I just finished one (+6 security) and it’s still showing low security for the system.
EDIT: today it switched to medium security \o/ and I also gained some population, even if there was no tick since last checked this night according to https://elitebgs.app/tick
May I ask what kind of gov is the controlling faction?
Democracy ✊
https://inara.cz/elite/starsystem/464318/
I think democracy was in the group with rather lower security. Meaning it most often will have medium/low