Processing.
Because lens blur does not look like that. The trick is to look at the highlights. Post process blur is uniform and smoothly dull. Real optical ens blur at highlights will often have chromatic aberrations and color fringing, and the highlight area 'comes alive' on its own, often taking on the iris shape/aperture shape of the lens, and that shape will vary depending on aperture set, and location on the frame, center of frame more round, edges of frame more oval, and often with starkier contrast at the edges and different highlight areas at different distances/planes will behave differently at varying amount of this effect. A post process blur on the other hand, will be uniform and on a single flat plane of the digital image.
To an untrained eye, you can't tell, but to a bokeh fanatic, it looks cheap. A more 'expensive' post process blur will be the Alien Skin Bokeh filter from Alien Skin, but its damn expensive and what it does its it improve the look of the highlights, but still far far from real optical blur.
