How to Figure "Partly Sunny" vs. "Mostly Cloudy"

Our friend Micky in Massachusetts recently asked for the difference between "Partly Sunny," and "Mostly Cloudy." The National Weather Service bases these terms on sixths of sky cover.
Here's the breakdown:
1) Sunny/Clear - no clouds, 0/6 clouds
2) Mostly Sunny - 1/6 clouds
3) Partly Cloudy - 2/6 clouds
4) Variably Cloudy - 3/6 clouds (Which is what this photo shows)
5) Partly Sunny - 4/6 clouds
6) Mostly Cloudy - 5/6 clouds
7) Overcast - 6/6 clouds