so how does averaging work?

Looking at BBC Weather forecast on my phone, I see it’s predicted to be nice and sunny this coming Wednesday:

When, though?  Mostly morning or afternoon, or maybe all day?  So I go to the detailed view:

Hmm.  I wonder what algorithm mangages to average this pattern of “light cloud” and “sunny intervals” to “sunny”?  Even not including the nighttime values, this seems a little optimistic.