DeltaV Reporter (Time) Average Value

I have searched but cannot find an answer on what the difference is between Time Average Value and Average Value when I use the Calculated Data Function on DV Reporter. Does anyone know the difference in calculations or where I might be able to find information on it? I have done side by side comparisons of the two and sometimes they are within tenths of each other and at other times there is a noticeable disparity. Also, I have noticed that, without any advanced filtering in the Data Overview, the Average Value function will return a N/A when a variable in a given time interval when there is bad/missing data but with the same parameters set the Time Average Value will return a valid number as if it glossed over any missing data, is there any rhyme or reason to that? 

Any help is appreciated!