Did you know EVERY PROTOSS unit can reduce the damage of an attack more than an ultralisk?
If you have a Protoss unit with +3 shields and +3 armour. The attack that breaks the shields of a Protoss unit will have it’s damaged reduced by both the armour and Shield values for most Protoss units that will be 7 damage reduction on that attack. With guardian shield that makes a 9 DAMAGE REDUCTION for that attack and if they have no base armour still 8 damage reduction.
Now knowing that a few other fun facts. Anti-armour missile increases damage by 6 instead of 3 while breaking shields because both shields and armour values get changed instead of just taking a flat increase in damage while Guardian shield does not reduce damage by 4 but only 2 on the attack because guardian shield is passive -2 damage from projectiles while not affecting armour values at all.
As long as there is at least 1 armour and 1 shield point this interaction will happen. While in game the chances of this dictating results is low, rare circumstances it might result in a unit surviving one additional shot during an engagement it is most noticeable with interactions with shield batteries/shield battery overcharge since this will take affect as long as shields are not depleted meaning a single protoss unit with a shield battery can be harder to kill than an Ultralisk.