I'd like to know where you got that information from or what specific parts, materials and their alloy specifications we're talking about. When a component is designed a specific material specification is called out on the engineering drawings and within that material specification is the alloying requirements. For example, if a component was to be made from 7075, it doesn't matter if the aluminum is bought from Reynolds or Alcola, 7075 is going to be made from "5.1-6.1% zinc, 2.1-2.9% magnesium, 1.2-2.0% copper, and less than half a percent of silicon, iron, manganese, titanium, chromium, and other metals"
The British would do the same on some of their earlier material specs for turbine engines and the only major difference you're going to find is the amount of impurities that may be in the raw state of the material. Using my example of 7075, as long as the ingot meets the basic alloying requirement for the spec, it doesn't really matter what supplier the material comes from.