Why is the boiling point of rectified spirit lower than alcohol absolute?
Ethanol and water have weaker intermolecular interactions than water with water and ethanol with ethanol. For this reason ethanol and water molecules can more easily escape from the liquid mixture, called azeotrope, to the gaseous phase than it is the case for the pure alcohol. Consequently the azeotrope has a lower boiling point.

What is considered as an impurity depends on your use case. E.g. 96% pure ethanol may qualify as extra fine spirit.