Seek the explanation of mathematical miracle, tell me the principle, or how to calculate it.

That's right. Pay attention to the area above you. The hypotenuses of the red triangle and the green triangle are not in a straight line.

I don't believe the tangent of an angle can be used for comparison. Red tana=3/8, green tanb=2/5. These two inequalities mean that they are not on the same straight line.

It means that there is a small area difference between the red one and the green one. It's just the extra piece under you.

Looking at the picture carefully, it is obvious that the hypotenuse is concave.

The bottom one is straight. That's all.