Many years ago, around the turn of the century, I worked for a company that processed glass for windows and other uses. The company was sold many years ago after becoming a supplier of Anderson patio doors (thanks to the impression I made loading glass into the washer during the inspection in negotiation 
). Before leaving due to a rotator cuff injury and pinched nerve in my neck I ran cnc “v-groove” machines that cut patterns into glass sheets.While most people may think glass is flat, there is often major deviations. Plastics are more deformable and if a sheet is clamped down for cutting the final product may exhibit unexpected deformation (to the end user) due to stresses in yhe sheet and thermal effect of the cutting process.