Looking at the example below for lib-tech.com, we can see that they are receiving an estimated 28.6 thousand visitors per month, on average based on how high their pages rank for keywords within Google's top 100 results. We can apply this same notion to the 175 estimated monthly visits from PPC ads we see under Paid Search.
When researching keywords, the volume listed represents an average monthly number of how many times this keyword was searched in the database selected (US Google Desktop in the image above).
To calculate the monthly average search volume, we take the keyword's search volume from the last 12 months starting with the current month or historical month selected in the report. Taking the average of the previous 12 months is also how we calculate CPC, Traffic %, and Paid %.
Since the Position Tracking tool updates daily, the search volume numbers in a Position Tracking report are not monthly averages. Search volume and traffic estimations displayed in a Position Tracking report represent daily estimates based on the specific locations you're tracking keywords in.