Daily scraps the data from https://rpi-imager-stats.raspberrypi.org/. rpi-imager-stats provides real-time stats of operating system/images downloaded for the Raspberry Pi using Raspberry Pi Imaging Utility.
For more information on rpi-imager-stats https://github.com/raspberrypi/rpi-imager/blob/qml/README.md#telemetry
Few call outs about the quality of the data.
- the source is in real-time
- the source is percentages
- the source != successful* installation
- a user might download multiple times, they might try a number of OS/images etc
Summary: it's a guide, educated guess not exact, perfect numbers.