It's an illusion, here's what happens: The two triangles (Red & Green) are not the same, that is they have different slopes. (If they were similar, the red triangle's sides would measure 3x7.5 not 3x8)
In the upper diagram the overall slope "bends" inwards and in the lower diagram it "bends" outwards. This difference in area is where the "hole" comes from.
Clear ?
AndyG
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