Most modern seamed stockings are made on circular knitting machines and include elastane, which allows them to stretch, recover, and fit a wider range of leg shapes. That’s what makes them practical and comfortable for everyday wear. https://www.whatkatiedid.com/collections/seamed-stockings
Original fully fashioned stockings were made differently — knitted flat, shaped, and then sewn together, and typically from 100% nylon with no stretch. That’s why they required a much more precise fit and behaved very differently when worn. https://www.whatkatiedid.com/products/fully-fashioned-stockings-point-french-heel