Author here.
My guess is some high-paying jobs that were called "Data Scientist" 3 years ago are now called "Data Engineer" and "Machine Learning Engineer", hence the average salary for jobs still called "Data Scientist" goes down a little.
Note the actual percent depends on how you measure it.
Full source/data/code: https://www.kaggle.com/nomilk/data-science-language-and-job-...