I've seen varying accounts on how status is applied to shields. Some say it is flast damage, others says if the swipe would have set you on fire, it deals the fire damage to the shield.

Perhaps it's a combination of both, and that's what we're seeing here? I find it very hard to believe that getting hit by different amounts of the hitbox will affect how much damage you take, and the only random factor on swipes I can think of is the status being applied.

Test with a fire resistant shield? I'll go do stuff with a level 1 savage tortoise and volcanic plate for comparison.

EDIT: @Dibs There seems to be a hitbox around vana's mace that applies knockback. For some reason this is slightly larger than the damaging hitbox, thus the non-damaging 'tink' when it hits a shield. I'm guessing that's what it is anyway, it's happened to me lots of times before, and I've seen people get knocked back in the same area as I was but curiously never noticed their shields reacting. Perhaps this is a clientside bug?

The only times I've seen it is with a stun-resistant shield though, so it could be applying just stun status. I don't know if vana's mace even does that. But I have a feeling taht SSB would survive a simple status appliance even if it doesn't have resistance to it.

Sorry this took so long, but parents interviened and there was a brief period of sleep between me getting these results and me actually being able to post them.

So I took a bunch of screenshots of my shield bar after being hit by the standard swipe attack of a slag walker, the one they do if you stand in front of them.

Here's how the savage tortoise performed

And here's volcanic plate shield

Interestingly there was not that much of a difference. these were both level 1, against a slag walker on the first depth of FSC (D24? can't remember)

Then we get the Nether shell results

As you can see, indeed there are varying results from the swipe.
I was testing different distances, angles, all sorts. Nothing gave a dependable result.
The bottom picture/shield bar just shows that you can be standing right up against that walker's face and it could deal the lower amount of damage.

So let's see what these shields have:
Savage tortoise: Normal, piercing, chock and poison. Only one that could have been useful there would have been normal.
Volcanic plate: Normal, stun, fire. Again, only normal and fire resistance would have been useful here.
Nether shell: Normal, shadow, stun and freeze. On normal and shadow would be useful here.

So from looking at these we can deduce that either shadow defence or 4* rating are the factors causing this seemingly random amount of damage done.

I have a feeling that enemies can deal a random amount of their specialized damage. Maybe this is elite exclusive, on normal they deal a constant?

EDIT: By the way, the idea of 'different amounts of hitbox being hit' is dismissed. I literally cannot see this game having a mechanic as complex as that. I've never seen a game that has a system like that in place.

Though, it's unlikely to be here. By that logic Swiftstrike should break several times over because of Silversap pounding. On Grey Owlite - there was a research that status damage multiplies overall shield damage, causing it to break much faster -> breaking Grey Owlite as it have no freeze resistance. As Swiftstrike, this is probably Sonic-Mode effect (that one that keeps you 1 HP).

As for collisions, [note that it's purely speculation and observation, no source code was involved] - there unlikely to be collision boxes. Game runs on tile-map and infinite height cylinders and sectors
If by logic "different amounts of hitbox", then damage on our weapons shouldn't be consisent either

Quick, someone call 1-800-GLACIES!