That's an interesting observation! I watched members of my cycling club unclip at a stop last weekend and most of them unclipped their right foot. When I asked about it the logic was that that was the foot they put "on the curb" when they were kids out riding their bikes.

I've always used my left hand for the bottle (because my right hand is much stronger and steadier on the bar) and unclip my left foot (because my right leg is much stronger for the push-off). I thought everyone did it this way!