There's a much simpler alternative, too: specular highlights often aren't just of the colour of reflected light, but they become affected by the surface colour of the item which is reflecting that light. As such, the _s texture might be simply RGB colour which is mixed with the colour of the light. At least that's how it was used in DAO and DA2 _s textures, iirc -- rgb channels would define tint of the specular highlight, and alpha would determine how strong the tint was.
If you have a look at the different channels, they dont represent the same data, red and blue appear to be masks of some type, the blue channel in particular only has two states, 0 or 255, Red appears to have different states, but consistent across a surface. Only the green appears to actually be a greyscale specular map. In PBR you usually have metallic and roughness to describe a surfaces properties, thats why I proposed Red as being a roughness mask, as I have not seen anywhere else this type of mask - Now frostbites PBR may be different, and I could be completely shooting down the wrong range, lastly the alpha channel is unused.