Funny, I'd always thought it was the opposite. I thought there used to be more different users on the site, making it easier to get the uniques as soon as you accumulated the number. It seems as if a core group (as knobby mentioned on his post) gives most of the thumbs recently, leaving reviewers short on uniques.
I did run some data out of curiosity. Here are the average number of thumbs on the first 30 reviews from some TRs - the first 5 diamond and then a selection of newer reviewers (2019 or more recent).
TVK - 10.4 thumbs per review
mashnut - 12.3 tpr
harr0140 - 11.0 tpr
optidiscic - 13.3 tpr
ERicJ - 14.2 tpr
lee76007 - 8.9 tpr
knobby325 - 3.8 tpr
Shadrach3 - 7.7 tpr
kp_1024 - 7.9 tpr
DFrah - 7.0 tpr
Of course, this could be badly skewed if enough people voted for the old reviews in the years in between.