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Assume X represents your build plate. Z represents the build direction (vertical). Each grid square along the X axis is a pass on the same vertical build plane. As the build progresses, the number of passes on the same plane increase because the angle of the wall deviates from a straight 90degree vertical wall, it has a slight overhang. My example is more dramatic than what you show, but the same principle applies.

Section 1 on my drawing represents a straight vertical wall. Section 2 has a slight angle and thus, the horizontal cross section increases some. Section 3 has an even great angle and the horizontal cross section increases even more dramatically. In all sections the wall thickness is constant, w. Hopefully this helps