facet average
 

I am computing the facet average of wall function heat transfer on a wall. The thing is that I am monitoring the facet average values, but when I check the facet averages using the surface intergrals I get different values. And when I check the max and min facet values compared to the max and min contour values they are slightly off. Why is this? I have checked the user's manual, but couldn't find a definite answer.