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Because you can't actually cut the pieces like that, though it's close. The angle of the triangles doesn't match the angle of the rectangles with slanted tops.
Rise over run of triangles = 3/8 = 15/40
Rise over run of angled pieces = 2/5 = 16/40
There would be a sliver of a gap along the main diagonal in the right-hand picture, enough to add up to one square's worth.
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