If you read the back label of the bottle, the alcohol is the active ingredient, and the remaining percentage is purified water, so any rubbing alcohol with less than 91% will evaporate a disproportionately large percentage of its alcohol, leaving the remainder with less and, potentially, eventually leaving only water behind. So the higher the concentration, the less water, which in turns reacts differently to the air in the atmosphere.
Just my 2 cents.