DiffImageStats

Field	What it counts
pixels	How many pixels the frame holds.
covered	How many of those at least one of the two pictures reaches.
unchanged	Pixels both cover and agree about.
changed	Pixels both cover and disagree about.
added	Pixels only the second picture covers.
removed	Pixels only the first one covers.
ratio	Everything that is not unchanged, as a share of covered.
distance	How far apart two pixels are on average, over the pixels both cover.

The four counts add up to covered rather than to pixels. Two pictures one of which is wider and the other taller leave a corner of the frame neither of them reaches, and those pixels are nothing at all rather than pixels that agree.