The Google/Facebook ratings are a lifetime rating. It is an average (mean) of the page rating (lifetime) for each unit in your system. We calculate that average based upon the most recent lifetime rating at the end of the time period reported on.
Ex. a report from Dec 1 - Dec 31, would take the most up to date lifetime rating from each page as of Dec 31. On the Trend report, that applies at the end of each month (Ex. rating on Dec 31, Nov 30, Oct 31, etc).