It's not due to the aperture blades. It's due to the amount of spherical aberration in the lenses.

Professional camera lenses carefully control this. It's actually one of the only intentional "flaws" added to lens. Technically bokeh with a slight ring on it (on point lights) usually results in a tack sharp lens. You can see this in most macro lenses.

Portrait lenses are sort of the opposite - they want super smooth bokeh (point light sources in the blurred out of focus area will have a nice gradient that is soft and gradually drops off toward the edges). This results in a lens that has slightly less sharpness on the in focus portion. It's not that noticeable though on a great lens.

Most lenses are a balance. When shooting professional cinema many lenses are intentional chosen for their bokeh qualities. "Nervous" bokeh is more on the side of rings and is often found on older Russian and East German glass.

There are two lenses where the amount of spherical aberration is not baked into the lens. They were made by Nikon called "Defocus Control". The 105mm 2.0 DC and slightly older 135mm 2.0 DC both have a second ring that looks another aperture ring but allows adjustability of the spherical aberration from "nervous" to "smooth". They were marketed poorly and misunderstood but are some of the best glass you can get - they test insanely well and have huge flexibility.