The scoring System is based on three factors which are then averaged to formulate a cumulative month score. The scores aim for a normal distribution between all months (ie: sun scores are not specific to sun in let's say September, they are general) and are meant to be thought of as a percentile(ie: a 60 means the place is better than 60% of months between all places). Average month scores above 90 are pretty rare and an average month score of 80 is actually quite good. Even down into the 60's represents generally enjoyable weather (60th percentile isn't bad!)
The sun score looks at the total number of average sunshine hours for a city in a given month and scores on a custom fitted curve where cities with under 150 mean sunshine hours for a given month get increasingly low scores whereas cities with over 300 hours all get very high scores at a decreasing rate (ie 400 hours is not considered that different from 300 hours, they both get a good score, but 200 hours get a much lower score than 300 and 100 hours get's less then a third of the score of 200 hours.
The rain score averages two seperate scores:
The Temperature is based on the daily mean temperature for a city in a given month and gives the highest scores for daily means between 65f and 70f but does not heavily penalize a city until it's average goes above 80f or below 55f. Note that night time temps mean that these numbers skew low. So if a city has a daily average of 85f it's day time high is likely between 95f or 100f.
Not all countries have all three scores calculated and accuaracy goes down when less than all three data points are present.