Yhink of the surface of the ground as a wire frame model.  The mesh resolution measures the distance of a side of the square that forms each element of the mesh.  At 76 meters... the corners of the "box" are 76 meters apart.  At 76 M, that is something like HALF the distance apart of the4 default mesh.

So of you have a small lump in the real world landscape that sits "between" the surroinding general terrain elevations..... it is "missed" as the average of the area is "drawn" from the 76 meter sample points.

I hope that makes some sense.   

best,

........................john