Is % of American population really a fair comparison? The fastest growing segment of the U.S. population is people over 65. Retirement takes many of them out of the workforce. This segment has grown by more than 15% since 2000 and the rate of growth is increasing more rapidly now.
Maybe the study accounts for that, I don't know. If it doesn't, then that is not really a fair representation either.
The proportions would have remained the same respective to the overall population. Hence, the reason we use percentages rather than straight counts.