My straps are made by hand so I can make a strap any width you like, however my straps are parallel width and I only keep a stock of buckles in 18mm, 20mm, 22mm, 24mm and 26mm. This is why, when you place an order, the options for strap width are limited to 18mm, 20mm, 22mm, 24mm and 26mm.
If you want a different width strap I can make it for you, but you will need to source a buckle for it yourself (or I can fit the next size up i.e. a 22mm wide buckle on a 21mm wide strap).