I can give you that answer. They started as a European company, and went with the metric system. And 30mm is a nice whole number. Any way you can increase the bearing surface for light to pass thru, the more brighter a scope is. But with todays coatings, you can almost perfectly match the index of refraction with air. Different air densities change with altitude and pressure, and humidity so they are not always perfectly matched. But the internal lenses should be constant.

Solitary is correct. Its a wash on a properly made scope that is adjusted to gather all the light possible from the objective lense. Problem is perfectly adjusted. You can have much more slop with a 30mm.