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I think the 'quality' of the bokeh increases as the distance to the subject decreases, and it's something I see with a number of the shots on here. It's when you have points of light in the distance that are slightly OOF but not totally blurred out.
The reason for shooting both lenses at their max. apertures was to judge the difference between them in 'subject separation' when the subject is in the middle distance. Clearly the Sigma is more successful at this.