Les wins the prize - except I don't have one.
It's because of the shutter on an SLR, which takes the form of a slit moving across the film or sensor. The reason that only some of the blades are curved is that a blade parallel to the slit is all captured at the same time and so appears straight. A blade at right angles to the slit is captured progressively as the slit moves across.
Somewhere I have a photo of a circular piece of card with diametric lines drawn on, which I whizzed around in the lathe and photographed. I can't find it right now - must be archived - but it shows the effect very well. Honest!
