If the triangle is 5 units by 13 units the overall area is 32.5 units. The hypotenuse does not intersect the unit by unit grid exactly, and when the red and green triangles are switched the areas are not the inverse of each other. the yellow area is also not exactly the same in both as it's upper left corner is truncated slightly along the hypotenuse in one. Drawing the triangles accurately (on graph paper or in CAD) will show that the differences between the colored areas area exactly a single unit.
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