Yep therein lies the answer. They are not perfect right-angle triangles. Put a ruler along the hypotenuse of the first "triangle" and it is not straight. The second triangle is nearly straight but not quite.
The clue is really in the website name at the top - "Subliminalmessages.com". Your eyes are seeing something different from what is there. Otherwise it is an impossible puzzle. If we say the the grid squares are 1 inch by 1 inch, then the area of each supposed triangle should be 32.5 square inches. But if you break each triangle up into its respective parts then the first triangle totals 30 square inches and the second one, including the "extra" square, totals 33 square inches. An absolute impossibility if they were both proper triangles.
And just to try and confuse everyone, the grids on the graph are not cms or fractions of an inch. Would appear that each grid is 12mm square as best one can measure on a computer screen.
Excellent puzzle.