I want to know the answer to the following question:
Why did the definition of film speeds change considerably?
All the background I'm provided with is that "This revision also saw significant changes in the definition of film speeds in order to accommodate then-recent changes in the American ASA PH2.5-1960 standard, so that film speeds of black-and-white negative film effectively would become doubled, that is, a film previously marked as "18° DIN" would now be labeled as "21 DIN" without emulsion changes.". Is that enough to answer the question?
Answer: yes