The current shield bonus AC of 25% was added as absorb armor.
This means a typical 36AC shield would give you 9 AC, for a total of 2.16% more absorb chance.
This AC did not benefit you for shield bashing & deflecting, it was absorb only.
The shield bonus AC is now 20% and is added to DEFLECT armor.
This means a typical 36AC shield will now give you 7.2 AC, for a total of 3.6% more DEFLECT chance.
This AC now benefits your shield bashing & deflecting. It also means the 3.6% added can deflect up to 100% damage, giving a little more tank for them.
So you will see a little LESS AC, but your overall chance to stop damage will be higher because it is now deflect armor, with all the perks that deflect armor gives Knights.