Icons not always matching with forecast

Weather Icons not always matching with the forecast: as you can see below, for the city of Rome, Italy you got "clear" both in the morning and in the night but the icon reports "partly cloudy" and not clear. Why?

fcttext = "Clear. High of 95F. Winds less than 5 mph.";
"fcttext_metric" = "Clear. High of 35C. Winds less than 5 km/h.";
icon = partlycloudy;
"icon_url" = "http://icons-ak.wxug.com/i/c/k/partly...;
period = 6;
pop = 0;
title = Sunday;
},
{
fcttext = "Clear. Low of 64F. Winds less than 5 mph.";
"fcttext_metric" = "Clear. Low of 18C. Winds less than 5 km/h.";
icon = clear;
"icon_url" = "http://icons-ak.wxug.com/i/c/k/clear....;
period = 7;
pop = 0;
title = "Sunday Night";