It's not that they deliberately made a math error, it's that it's a very crude algorithm that basically just blurs everything that's not within what's deemed as the subject with some triangular, Gaussian, or other computationally simple kernel.

What real optics does:

- The blur kernel is a function of the shape of the aperture, which is typically circular at wide aperture and hexagonal at smaller aperture. Not gaussian, not triangular, and the kernel being a function of the depth map itself, it does not parallelize efficiently

- The blurring is a function of the distance to the focal point, is typically closer to a hyperbola; most phone camera apps just use a constant blur and don't even account for this

- Lens aberrations, which are often thought of as defects, but if you generate something too perfect it looks fake

- Diffraction effects happen at sharp points of the mechanical aperture which create starbursts around highlights

- When out-of-focus highlights get blown out, they blow out more than just the center area, they also blow out some of the blurred area. If you clip and then blur, your blurred areas will be less-than-blown-out which also looks fake

Probably a bunch more things I'm not thinking of but you get the idea