Can someone explain why this happens?
Also - interestingly (from Wikipedia): "Bokeh can be simulated by convolving the image with a kernel that corresponds to the image of an out-of-focus point source taken with a real camera. Unlike conventional convolution, this convolution has a kernel that depends on the distance of each image point and – at least in principle – has to include image points that are occluded by objects in the foreground. Also, bokeh is not just any blur. To a first approximation, defocus blur is convolution by a uniform disk..."