square or rectangle ? Envisioning Information, page 84

I am reading the “Envisioning Information” (eighth printing). On the right side of page 84, there is a illustration of the “Pythagorean” theorem from the classical Chinese book “Chou Pei Suan Ching”. The small center square in the induction equation looks more like a rectangle to me. Is this a mistake in the re-drawing or a visual effect? (I attempted to measure with a ruler, it seems the vertical is shorter than the horizontal, but not sure)

I am more curious what contributes to such a visual impression if it is indeed a true square

Thanks.

-Xiaoying Lin