I can't look at the fine print of the policy right now, but they wat bannerlord stuff usually works is if its says "+10%" it means something way suckier that's technically +10% like that one might be "you would get +1 militia a day but now you get +1.1 a day..... every 10th you will have an extra 1...that 10% more!"
Okay I looked at it
Charter of Liberties
Militia quantity is increased by 10% and 10% of militia troops will be higher tier.
Revenue paid to both liege and local ruler reduced by 5%
I don't know that wording is weird like you would expect a lump sum increase of militia, but I doubt that happens. I wish we has a bannerlordperks for policies to see how they actually work.