This is because they are used for different compression grades. (compressioncarl.com)
15 - 20 mmHg Support Therapy Collection Compression Stockings available in different compressions and styles: Knee Highs, Maternity Pantyhose, Pantyhose and Thigh Highs. (legstherapy.com)
I fly a lot and I'm more than a little paranoid about DVT on longer flights, so I decided to test out a bunch of different compression socks. (armchairempire.com)