When optical technicians fit eyeglass lenses into plastic eyeglass frames, do they purposely adjust the frame shape in order to fit the lenses in?

To those who wear and have purchased eyeglasses——Is it me or do optical technicians sometimes adjust the shape of the eyeglass frame in order to fit the lenses in? In my experience, I have always been surprised when I get my new glasses——the shape of my new plastic eyeframes doesn’t seem to be the same shape as it was when I first tried them on in the eyewear shop. It’s frustrating, for example, when a very rectangular frame becomes a “bit oval” in shape, not what I wanted. Why do optical technicians do that? Can’t they make lenses to fit into the rectangular frames I originally wanted?