Answer
What is the difference between a waiver and a discount?
A waiver removes the whole article processing charge. A discount removes part of it, most often 50%. Publishers typically run both tiers: a full waiver for one group of countries and a partial discount for a second. IEEE is unusual in graduating its discounts from 25% to 50% by country rather than using a single rate.
Detail
Why the tier you fall into varies by publisher
The tiers are drawn from different classifications. For 35 countries, Wiley grants a full waiver where Taylor & Francis grants 50%, for the same author and the same paper.