Consider a simple example below.
We have a metal sheet and on top of it we have blood splatter.
Some part of the dried blood splatter is present in grainy form.
Also the the blood splatter layer inside photoshop is set at 20% opacity.
Blood splatter brush-https://www.dropbox.com/s/s7o3znxldq0r6m9/Blood_Splatter.abr?dl=0
So,how will you assign the correct spec value to blood splatter.In order to render correctly.
In reality there is no such thing as partially present.Either something is present or not.
So when i set the opacity of dried blood at 20 %.Blood splatter is still there,its just not that intense.
Every object has a fixed spec value.Dried Blood also has a fixed spec value (Which i don't know.Take whatever you want in the non metal range).
In this case we should be getting either dried blood or metal spec value.
If we are getting a spec value which does not represent the blood or metal .Then the result is incorrect.