Do you have a source/citation on this? Not doubting you.. just find this very interesting...
Crystal makes sense as it offers both more clarity and greater light gathering capability (prism effect, which normal "glass" does not have)..
But...
All of the big manufacturers of German/Austrian optics get their glass from the same place.. Schott (which happens to be owned by Zeiss)..
All crystal is in fact "glass".. but not all glass is "crystal"... the principle difference in the two is crystal has a very high lead content (the EU mandates at least 24% for glass to be declared "crystal".. but most of the high quality crystal manufacturers like Waterford in Ireland have a much higher lead content.. Waterford uses a 32% blend..
Low lead content makes glass "soft"...high lead content makes glass more brittle than you would want in an optic.. (a true EU standard crystal I would think would be too brittle to stand up to recoil of many centerfire rifles?)...
Each company (swaro, leica, zeiss, etc) can choose whatever Schott glass/crystal formula they want.. so quality can certainly vary from one company to another or even one line from one company to another line from the same company.. So it is possible that one company is buying "glass" for their entry level scopes, and then moving up to "crystal" for their higher end lines, etc..
On Schotts website they speak to the "glass" they provide for binos, thermals, scopes, etc.. but I cant find any reference to "crystal" being used..
Click here to discover SCHOTT’s advanced range of sports optics offering products such as HT glass for long-range vision and optical prisms for high powered binoculars
www.schott.com
Schott actually only makes a very small amount of "crystal" from what I can tell.. most of which is used by their subsidiary Zwiesel to produce high quality stemware, bowls, and other household items (sold at places like Willams Sonoma, etc)..