Probably because 2.5 cm and 3.5 cm sounds better than 0.98 inches and 1.38 inches. And while ThinLine is an American company, they sell globally. Most of the world use the metric system.
I bought two of the FF muzzles, thinking that two of my horses could benefit from them. I ended up only ever using them on one mare. I cut the hole bigger, it probably has a diameter of 2.5". She wore that one season. I hand grazed her with it on for a few days until she figured it out and the frustration wore off. I imagine if I had of just put the small hole on and threw her out that she may have gotten it off.
Last summer she was on stall rest and small turnout for nearly six months. When she finally did get to go back out on grass she was fat and it was autumn, perfect recipe for laminitis. So I put her out with the uncut 2.5cm FF and had no issues.