I know in my case, it is an approximation or rounding off of the exit diameter of the reflector which is typically very close to the largest OD of the reflector.
Many reflectors have a flange around the large opening so that it rests on the end of the head tube. this increases the outside diameter of the reflector. The specified diameter of a reflector is the inside diameter , or optical diameter of the big end. See one of McGizmo's mechanical drawings for reference: http://dmcleish.com/CPF/reflectors/20pt6-reflector-mm.pdf
This is a 20mm reflector; the McR20. This reflector does not have a flange, so the optical diameter is the same as the overall diameter (or nearly so).