"Income growth of top 1%" is a very misleading concept. It sounds like "top 1%" is a fixed group of people, and we are talking about their income growth. Actually it is not that case at all. It is the "growth" of income from "top 1%" at different time points. Because it is about different group of people from different time points, it is misleading to call the difference as "growth". It's like last year best student A got score 100, and this year best student B got score 110, and you call it the "achievement growth of best student".

On the other hand, "income growth of bottom 99%" has the same issue, but it is less the problem, because 99% is a much more stable group of people. It's like "achievement growth of all students (except the best one)".