There's a misconception that the unit 'Bar' had been derived from (or interpreted to mean) 1 atmosphere of pressure. It's more of a coincidence that 1 Bar = 1 atmosphere (slightly less that 1 atm actually).
The unit 'Bar' is actually based on the MKS (Meter-Kilogram-Second) system of metric measurements. The root units from which 'Bar' is derived, is the gram and centimeter.
Since the unit 'gram' was chosen to represent the mass of a cc of water (under a set of strict conditions), there is no absolute mechanism which characterizes (defines) the relationship of 'Bar' to atmospheric pressure.