If the firm is designing the glass then it is the firms glass, if they buy finished products it is the manufacturers design, It must be that way or, for example, all of Dresser's glass is no longer Dresser. As to many later products of Edinburgh Crystal and Waterford were actually made overseas. Then a lot of glass registered in the earlier 20th century by UK factors is often sold under the factors name. It is an added bonus to track the producing factory too, but essentially they were just fulfilling a contract. China remains a red flag to many but is an emerging force in glass under its own banner.
Also as people buy/collect based on design then the age of the piece starts to become less relevant.