I lost my original post, so I'll try again, more briefly:
The different COEs probably originated by region and purpose.
The Romans worked with glass after bringing the technology from the Middle East. A glass with COE 96 ended up being used for furnace work and COE 104 for lampwork. Originally lampworking torches/burners were some fuel and air, so perhaps the glass with a lower melting point was a later development. There is a book, The chemical history of glass that explains how the different raw materials available affected the composition of glass. (I don't remember the author, but the publisher is Springer.)
Bullseye manufactured stained glass, and worked with glass artists to develop a line of glass that fused together reliably. Their formulations ended up with a COE of about 90. Bullseye doesn't prefer to talk about matching COE, but rather about compatibility, which takes into account the coefficient of expansion, viscosity, and other characteristics of the glass.
Spectrum and Uroboros also made stained glass. Spectrum also made furnace glass, so when fusing became popular, they probably chose to continue with COE 96. Later, Spectrum and Uroboros came up with the "System 96" standard.
I can't find a history of Satake glass (COE 113-120), but I suspect it is a later development to make flamework easier to accomplish with the Japanese air-gas burners. I believe the lower melting point is achieved by a greater percentage of lead in the flux.
I have not found why Schott chose to use a formulation with COE 91. It was probably developed for laboratory glassware, before the development of borosilicate glass.
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