Yep, this—we aren’t as close to the equator, but there’s less atmosphere to filter UV. Not so much in terms of beach days, of course, so sunburns here tend to be less than full-body.
All that said: a daily mean of 13-14 is past what we’d see here—and yeah that is some serious burning.
My area is similar to Sydney in your map. For Denver it’s the elevation that’s exposing them to more UV rays.
[1]https://www.epa.gov/sunsafety/sun-safety-monthly-average-uv-...
[2]https://www.reddit.com/r/MapPorn/comments/cd1t54/relative_la...