A reflectance greater than one is caused by a conductive object traveling through a magnetic field. Due to its great speed, it is a massive generator, and the object takes on a very high voltage charge. When that voltage leaks away in a coronal discharge, it generates light. Kinda like a weak arc lamp.
This causes it to be brighter than simple reflectance.
Ah, thank you. That explains the greater than one. I still don’t see how one gets a less than zero value.