In Fluent we have the option for mass weighted and area weighted averaging. For a flow boundary, let's assume the bulk of the area is at a low velocity and a tiny fraction (1/10 of area) is at a very high velocity (x10 speed). nArea average for temperature will bias the value to the low velocity region as it's the bigger area. That would be wrong. nMass average would calculate the mean temperature based on how much mass was at what temperature.