Plastic cuvettes are cheaper than quartz, for one! Did they mix the fluorescent dye in the plastic, too? I'd have to see what they were measuring, but you do know that plastic adsorbs certain materials that quartz will not. Perhaps there was some fraction of the saliva solution that they did not want transferred over. When doing antibody research, we routinely used plasticware to make the antigens stick to the plastic. That is a possible explanation.
