I think it's the latter; looking at the 2017 ADA DAT guide higher scores are becoming more and more common. My 24 used to be in the 99 percentile but now is 97ish. Personally, I think in the near future a 22 will be the average accepted applicant score. I was so excited when I got my score but then I realized that in reality it's nothing special. I was even thinking of re-taking the DAT and try to score higher but I realized that it is more likely I end up doing worse or get the same score and I shouldn't be greedy and try to get a higher score.
*EDIT: The study is how many exams taken, not # of applicants but still I think it's getting higher; a study that only looks at applicants still show an increase in DAT and GPA for the average accepted applicant but it is a lot more gradual but doesn't really show data on DAT percentiles.
U.S. Dental School Applicants and Enrollees, 2016 Entering Class. One thing that's interesting about the study is how for GPA ranges 3.25-3.49, 3.5-3.75, 3.75+, the proportion of students enrolled with this stats are similar around 24% each but for DAT 19-20 and 21+ combined make up 80% of the class on average which to me clearly illustrates DAT> GPA as if you have anything below a 19 your chances diminish significantly but for GPA there is a wider range of acceptable GPAs.