The data comes straight from Google's Ad Center (myadcenter.google.com). Google shows you the interest categories and brands they've assigned to your profile. I automated scraping that page daily for each account during the experiment.

MirrorMask actually does exactly what you're describing. It scrapes your Ad Center profile before and after each session and shows you the diff. You can watch interests appear and disappear over time. The dashboard tracks your profile changes across sessions.