Because the anamorphic setup affects blur circles in two ways. The anamorphic element squeezes the image in one direction, and that squeeze is later reversed in post or projection, so in-focus subjects look normal again. But out-of-focus highlights are also shaped by the lens opening itself. A normally round aperture or lens barrel appears oval when viewed through the anamorphic element, so the blur discs are effectively altered by the optics and the aperture shape.

In other words, the bokeh isn’t just a “normal image” waiting to be un-stretched; its shape was formed by the optical path. After de-squeezing, the scene geometry is corrected, but the blur highlights still reflect that oval effective aperture, which is why anamorphic bokeh is characteristically elliptical.

This is similar to how masking the front of a lens can change bokeh shape: the aperture shape influences out-of-focus highlights. With an anamorphic attachment on a round lens, the system acts like it has an oval effective opening, leading to oval bokeh.