The reflection of the interface of two layers is described by Fresnel's equations. For instance, the reflectance of an Al bulk at 1 micron is around 0.9, this is the reflectance of the interface between air and Al, and it shouldn't change no matter how thick the Al layer is, as long as the interface is still the same. Now assuming we polish the Al to a 1 nm thin film from the other side, and keep the air-Al interface intact. Then the reflection can be lower than 0.9. The refractive index of Al doesn't change, and the smoothness of the interface is still the same, why the reflectance can be different?
I believe 1 nm is of the order of magnitude of the skin depth for visible light in aluminum, so part of light gets to the farther surface of the Al layer and partly exits the layer, so the reflectance decreases. Remember that reflectance typically depends on the thickness of the layer.