I'm telling you, if you take a spherical object at random, you will see a "pure" circle from all angles, while the oblique double-sided drawing is to draw a three-dimensional figure intuitively, so the ball should still be round.

However, if you have studied the knowledge point of perspective in art, you will know that it is wrong to slant both sides. But we ignore this difference in mathematics)

Asking such a question, I guess you are a girl.