I'm with you on wishing there was a link to the actual research. As a researcher myself, I'm rarely satisfied unless I can get my hands on a methodology report.
Having said that, it does make sense to me that % increases would be higher earlier in the career. Going from 0 experience to 1, 2, 3, 4, 5 years experience is a huge increase in your worth to a company.
However, I think a top performer is still going to see nice increases later in their career. Remember that 3% of $30k is not as much as 2% of $60k in terms of $$ increase.