it could be called a 'true' average, but depending on your camera, it is limited. It does not look at the
light levels etc at every pixel and do the calculation, rather it takes a sample at various points across the scene and uses those. How many points and where those points are depends on your camera body and what the manufacturer decided to setup as the evaluative
metering points.
So whilst it uses more points than spot and centre weighted, it doesnt use the entire scene as a whole, rather a wide spread selection. So calling it True is probably not 100% correct, but maybe multi point
metering would be better.
Nikon call it Matrix
metering cause it use a matrix of points to work out the average, not the whole scene as such. In most new
Nikon DSLR it uses 1005 points in the scene.
So looking through your viewfinder consider how 1005 points (that are evaluated to determine the correct settings) would cover what you see. So it is fairly damn comprehensive, but it is not perfect, then nothing is.