MGS Mechanics quiery (in General)

Another thread triggered rememberence of a slight quirk in mechanics I can never really remember being explained.

When facing small DD damage values how *does* the MGS reduce incoming damage to zero? On minion with no other AC, this should be impossible, as the MGS gives no natural AC (or direct reduction) itself.

:/

It gives a direct reduction in itself something small like (IIRC) 300 or so, very small, but really really noticeable down low.

each point reduces something like 150 damage directly, like a point in AC reduces damage a little bit directly.

But, why?

It was never revealed to have any direct reduction.

Is that an intended feature (which should probably be documented), or an unintended bug?

It's been known for many years, probably should be documented.

Oh I agree it's negation of small damage values has been known for, ages.

But is it a bug?

Or are we assuming that as there's never been any word on this in the past, we should just accept it as given?

And do we know how much the direct reduction is? It seems significantly larger than an equivalent amount from AC.