How are "Best Times to Call/Email" Calculated?

How are "Best Times to Call/Email" Calculated?

I am not seeing any real precise algorithm being used to calculate the "Best Time to Call/Best Time to Email" field (see  Screenshot below). It seems to be all over the map. Can anyone explain how this is calculated? I'm needing to know this because the Workflow Rules allows for an email to be sent at the next "Best Time." But it seems to fail if the CRM has calculated "no best time."