As someone who has resided both in a foggy part of San Francisco and in Portland, I feel that this index doesn't adequately capture the dreariness of some SF neighborhoods.
*"300 days of sunshine per year" is frequently cited, even on the official Visit Denver website (https://www.denver.org/meetings/denver-info/weather/). Having lived in Denver for the last ~15 years, it is very sunny, but "300 days a year" stretches the truth just a bit.
It seems that 100 or so years ago, promoters decided that was the magic number to attract people.