I've placed a MIRROR into my system, but I haven't defined a coating for it. How is its reflectivity defined?
If a surface or object has the material type MIRROR, and no coating is specified, then the surface is assumed to be coated with a thick layer of aluminum, with an index of refraction 0.7 - 7.0i. The aluminum layer is assumed to be thick enough that no light propagates past the layer.
This means that an uncoated mirror surface has a reflectivity of less than 1, just as an uncoated refractive surface has a transmission of less than 1.