Bob,(posted 8159 days ago)Don't confuse ISO with anything to do with the real world. ISO standards apply to various manufacturing and technical processes. There's an ISO standard for just about everything from what constitutes a 60 watt light bulb to how the wires in Cat 5 cables are twisted. They are there to ensure consistancy and nothing else.
So to your point.... with ISO standards I know for certain that a film with an ISO speed of 100 requires more exposure than a film with an ISO speed of 400. Nothing more or less. To actually find how I should expose the ISO 100 film to get a negative with sufficiant shadows and printable highlights is an entirely different matter. That's based on my own personal practices and equipment.
And to your point about a true 100 speed film - 64 to 100 is only 2/3 of a stop. In the grand scheme of things that's not much of a difference anyway. You can shoot Plus X -- rated at 125 by Kodak but with a true EI between 80 and 100.