I think it probably has something to do with eyelid strength. Most people have one eye that's slightly stronger than the other. For instance, hold your finger up in from of your face in the centre and wink with one eye and then the other. There probably won't be much difference in what you're seeing but there should be a slight difference with one eye, and the one with the change is the 'weaker' eye. For me, my right is my strongest.
It seems however that this doesn't just reflect eye strength but also eyelid strength, since my right eye lid is also stronger than my left. It took me a couple of months of practising before I learned to wink with my left eye. I looked in the mirror and noticed that my left eyelid fell faster than the right, probably because my right eye is stronger and I therefore can lower it slower than the weaker left eye.
Or that could just be me...
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