My guess is that if Quantum Shielding makes the armour value 13 on a vehicle, up from 11, that a lance weapon would be facing armour value 12 with the shielding up, and then back down to armour 11 after the shielding is taken down.
Quantum shielding as we know it now, adds a +2 to all the armour values of the Necron vehicle on all sides. It lasts until a penetration roll is made against it, which takes down the shields.
So does anyone have any more information on this?