Please let us know what you are expecting. The calculation in use is:
IF ISNULL(AVG([Outcome (copy)])) THEN
STR (ROUND(AVG([Outcome (copy)])*100,2))+"%"
The Round() function is used here to round to 2 decimal places. The Avg is per Measure Name but remember that this is not the average per month but the average of all rows of data. If you want to average per month then a LOD calculation might be better. Let us know.
Also - please remember to post the version number of Tableau that is in use when posting a workbook so others are able to open and save in the same version.