Is there a reason for this to be inverted?
The reason is obvious if you combine it with texture overlay for a texture with a View mapping.
Now, pattern of texture in center of brush where strength is the highest becomes invisible.
You just have a black spot in the middle that does not correspond to what will be sculpt. And you can only notice texture on border where strength is so small that texture pattern is not noticeable when drawn.
You should modify texture overlay too, to be multiplied by that alpha when both are enabled, instead of current addition.