Currently the average function in the Report Visualization treats null values as zeros, this is not ideal as nulls usually indicate value is unknown or unset, as such they should be ignored in the average calculation, or the average calculation should itself return null.
Alternatively Asana calculations could work like salesforce functions having an option as to whether or not nulls should be treated as zeros.
Steps to reproduce:
Create any report containing a numeric field (san numfield) that has some values unset (ie blank), create a visualization of the report, choose a grouping, and then choose to show the numfield: average field value.
Note how the calculation treats the null fields as if they had the value 0.